Currently the Internet communication is under threat of attacks, such as unauthorized accesses, message modifications and eavesdroppings. In order to guard it against these attacks, several mechanisms and protocols have been proposed in recent years. They were designed to perform their security functions in network and application lay- ers. However, providing security functions in the transport layer give us many useful features, thus we propose a new security mechanism called "Se- cure TCP" in the transport layer. This mecha- nism can be one of the basic units to form trusted network applications. In this paper, we discuss design and implementation of the Secure TCP.