diff --git a/.gitignore b/.gitignore
index bf30cc9..20d092c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -60,3 +60,16 @@ venv/
 ENV/
 env.bak/
 venv.bak/
+
+# OS generated files 
+.DS_Store
+.DS_Store?
+._*
+.Spotlight-V100
+.Trashes
+ehthumbs.db
+Thumbs.db
+
+# Compiled Source
+.pytest_cache
+__pycache__
\ No newline at end of file
diff --git a/contracts/accumulator/PrimeTester.vy b/contracts/accumulator/PrimeTester.vy
new file mode 100644
index 0000000..1a3780e
--- /dev/null
+++ b/contracts/accumulator/PrimeTester.vy
@@ -0,0 +1,89 @@
+# @dev PrimeTester
+# @author Nick Hyungsuk Kang (@hskang9)
+# Prime Tester using Fermat's primality test for a=2
+
+# TODO: Move constants to separate contracts after inlines are implemented
+### CONSTANTS ###
+# FIXME: The sizes of arrays in this file should be replaced with these constants:
+#        https://github.com/ethereum/vyper/issues/1167
+
+N_LIMBS_LENGTH: constant(int128) = 8
+G: constant(uint256) = 3
+
+M_LIST_LENGTH: constant(int128) = N_LIMBS_LENGTH
+M_BYTE_COUNT: constant(int128) = 32 * M_LIST_LENGTH
+M_BYTE_COUNT_BYTES32: constant(bytes32) = convert(M_BYTE_COUNT, bytes32)
+# For now, the same lengths are used for the simplicity of impelementation.
+BASE_BYTE_COUNT_BYTES32: constant(bytes32) = M_BYTE_COUNT_BYTES32
+E_BYTE_COUNT_BYTES32: constant(bytes32) = M_BYTE_COUNT_BYTES32
+
+PRECOMPILED_BIGMODEXP: constant(address) = 0x0000000000000000000000000000000000000005
+
+# Base for Primality test
+a: constant(uint256) = 2
+
+
+@private
+@constant
+def _compare(_a: uint256[8], _b: uint256[8]) -> int128:
+    for i in range(M_LIST_LENGTH):
+        if _a[i] > _b[i]:
+            return 1
+        elif _a[i] < _b[i]:
+            return -1
+    return 0
+
+# TODO: Remove arithmetic functions once BigInt.vy is developed
+### BIG INTEGER ARITHMETIC FUNCTIONS ###
+
+# TODO: Consider adding this function to BigInt.vy
+@private
+@constant
+def _convertUInt256ToUIntList(_uint: uint256) -> uint256[8]:
+    uint256List: uint256[8] = [0,0,0,0,0,0,0,0]
+    uint256List[7] = _uint
+    return uint256List
+
+@private
+@constant
+def _convertUInt256ListToBytes(_inp: uint256[8]) -> bytes[256]:
+    # FIXME: Make it more simple when conversion to bytes is supported:
+    #        https://github.com/ethereum/vyper/issues/1093
+    tmp: bytes32[8]
+    for i in range(M_LIST_LENGTH):
+        tmp[i] = convert(_inp[i], bytes32)
+    return concat(tmp[0], tmp[1], tmp[2], tmp[3], tmp[4], tmp[5], tmp[6], tmp[7])
+
+@private
+@constant
+def _convertBytesArrayToUInt256List(_inp: bytes[256]) -> uint256[8]:
+    out: uint256[8]
+    for i in range(M_LIST_LENGTH):
+        out[i] = convert(extract32(_inp, i * 32, type=bytes32), uint256)
+    return out
+
+@private
+def _bigModExp(_base: uint256[8], _e: uint256[8], _m: uint256[8]) -> uint256[8]:
+    base: bytes[256] = self._convertUInt256ListToBytes(_base)
+    exponent: bytes[256] = self._convertUInt256ListToBytes(_e)
+    modulus: bytes[256] = self._convertUInt256ListToBytes(_m)
+    # ref. https://eips.ethereum.org/EIPS/eip-198
+    # 864 = 32 * 3 + <length_of_BASE> + <length_of_EXPONENT> + <length_of_MODULUS>
+    data: bytes[864] = concat(BASE_BYTE_COUNT_BYTES32, E_BYTE_COUNT_BYTES32, M_BYTE_COUNT_BYTES32,
+                    base, exponent, modulus)
+    # NOTE: raw_call doesn't support static call for now.
+    res: bytes[256] = raw_call(PRECOMPILED_BIGMODEXP, data, outsize=256, gas=2000)
+    return self._convertBytesArrayToUInt256List(res)
+
+# this assumes that the number is less than 2 ** 64
+@public
+def isPrime(_num: uint256) -> bool:
+    assert _num < 2 ** 64
+    if _num < 2:
+        return False
+    det: uint256[8] = self._bigModExp(self._convertUInt256ToUIntList(a), self._convertUInt256ToUIntList(_num-1), self._convertUInt256ToUIntList(_num))
+    one: uint256[8]  = self._convertUInt256ToUIntList(1)
+    if self._compare(det, one) == 0:
+        return True
+    return False
+    
diff --git a/tests/accumulator/test_primetester.py b/tests/accumulator/test_primetester.py
new file mode 100644
index 0000000..effb4ff
--- /dev/null
+++ b/tests/accumulator/test_primetester.py
@@ -0,0 +1,37 @@
+import pytest
+from web3 import Web3
+
+from eth_tester import PyEVMBackend, EthereumTester
+
+
+N = [2,3,5,7] # primes less than 2**64
+
+a = 2 # base for primality test
+
+@pytest.fixture
+def c(get_contract):
+    with open('../contracts/accumulator/PrimeTester.vy') as f:
+        code = f.read()
+    # FIXME: eth.exceptions.OutOfGas: Contract code size exceeds EIP170 limit of 24577.
+    c = get_contract(code, N)
+    return c
+
+
+def test_init_state(c):
+    assert uint256_list_to_int(c.a) == a
+    
+def uint256_list_to_int(l):
+    out = 0
+    for i in range(len(l)):
+        out += l[i] * 2 ** (32 * i)
+    return out
+
+
+
+"""
+TODO: Set EVM Backend to increase its max_available_gas
+"""
+def test_prime(c):
+    assert c.isPrime(2) is True
+    
+