diff --git a/include/mcl/ec.hpp b/include/mcl/ec.hpp
index b3350b9..9b09486 100644
--- a/include/mcl/ec.hpp
+++ b/include/mcl/ec.hpp
@@ -686,7 +686,7 @@ public:
 		}
 		EcT P(*this);
 		P.normalize();
-		if (ioMode & IoSerialize) {
+		if (ioMode & (IoSerialize | IoSerializeHexStr)) {
 			/*
 				if (isMSBserialize()) {
 				  // n bytes
@@ -712,7 +712,11 @@ public:
 					}
 				}
 			}
-			cybozu::write(pb, os, buf, n + adj);
+			if (ioMode & IoSerializeHexStr) {
+				*pb = mcl::fp::writeHexStr(os, buf, n + adj);
+			} else {
+				cybozu::write(pb, os, buf, n + adj);
+			}
 			return;
 		}
 		if (isZero()) {
@@ -749,12 +753,18 @@ public:
 #else
 		z = 1;
 #endif
-		if (ioMode & IoSerialize) {
+		if (ioMode & (IoSerialize | IoSerializeHexStr)) {
 			const size_t n = Fp::getByteSize();
 			const size_t adj = isMSBserialize() ? 0 : 1;
 			const size_t n1 = n + adj;
 			char buf[sizeof(Fp) + 1];
-			if (cybozu::readSome(buf, n1, is) != n1) {
+			size_t readSize;
+			if (ioMode & IoSerializeHexStr) {
+				readSize = mcl::fp::readHexStr(buf, n1, is);
+			} else {
+				readSize = cybozu::readSome(buf, n1, is);
+			}
+			if (readSize != n1) {
 				*pb = false;
 				return;
 			}
diff --git a/test/ec_test.cpp b/test/ec_test.cpp
index ae42c67..a78357e 100644
--- a/test/ec_test.cpp
+++ b/test/ec_test.cpp
@@ -295,7 +295,7 @@ struct Test {
 			ss >> Q;
 			CYBOZU_TEST_EQUAL(P, Q);
 		}
-		// IoSerialize
+		// IoSerialize, IoSerializeHexStr
 		const size_t adj = Ec::isMSBserialize() ? 0 : 1;
 		P.set(x, y);
 		{
@@ -305,12 +305,24 @@ struct Test {
 			CYBOZU_TEST_EQUAL(P, Q);
 		}
 		{
-			P = -P;
+			std::string s = P.getStr(mcl::IoSerializeHexStr);
+			CYBOZU_TEST_EQUAL(s.size(), (Fp::getByteSize() + adj) * 2);
+			Q.setStr(s, mcl::IoSerializeHexStr);
+			CYBOZU_TEST_EQUAL(P, Q);
+		}
+		P = -P;
+		{
 			std::string s = P.getStr(mcl::IoSerialize);
 			CYBOZU_TEST_EQUAL(s.size(), Fp::getByteSize() + adj);
 			Q.setStr(s, mcl::IoSerialize);
 			CYBOZU_TEST_EQUAL(P, Q);
 		}
+		{
+			std::string s = P.getStr(mcl::IoSerializeHexStr);
+			CYBOZU_TEST_EQUAL(s.size(), (Fp::getByteSize() + adj) * 2);
+			Q.setStr(s, mcl::IoSerializeHexStr);
+			CYBOZU_TEST_EQUAL(P, Q);
+		}
 		P.clear();
 		{
 			std::string s = P.getStr(mcl::IoSerialize);
@@ -319,6 +331,15 @@ struct Test {
 			Q.setStr(s, mcl::IoSerialize);
 			CYBOZU_TEST_EQUAL(P, Q);
 		}
+		{
+			std::string s = P.getStr(mcl::IoSerializeHexStr);
+			CYBOZU_TEST_EQUAL(s.size(), (Fp::getByteSize() + adj) * 2);
+			for (size_t i = 0; i < s.size(); i++) {
+				CYBOZU_TEST_EQUAL(s[i], '0');
+			}
+			Q.setStr(s, mcl::IoSerializeHexStr);
+			CYBOZU_TEST_EQUAL(P, Q);
+		}
 	}
 	void ioMode() const
 	{