The NAT-PT, one of the IPv6 transition mechanisms, supports IPv6 node in a NAT-PT domain to communicate with IPv4-only nodes outside. However, due to the IP datagram conversion at the NAT-PT server, NAT-PT node has a problem in establishing the end-to-end security using the IPsec IKE and AH mode. This memo describes the reason why the problem is caused and proposes a solution for assuring the end-to-end security using IPsec in the NAT-PT environment.