In spite of the fact that South Korea and Japan are as much neighbouring countries across the sea as allies to the US，for long the security cooperation between them has been stagnant.The lack of sufficient political trust has its roots traced back to historical differences and island disputes.By the same token，the Korean government has been cautious in carrying out security cooperation with Japan due to the strong presence of anti-Japan sentiments domestically.The emergence of nuclear threat from North Korea has changed the security balance in Northeast Asia，where this threat is perceived acutely by both South Korea and Japan，as they share wide interests in terms of responding to the nuclear threat from North Korea and maintaining peace and stability in the region.Henceforth，the Korean nuclear issue has become an opportunity for both countries to bring down the barriers in security cooperation.The continuing development of this issue equally has become an important force to drive both countries to carry out cooperation in security.