Abstract: The present invention is to provide a method for establishing TCP connection according to NAT (Network Address Translation) behaviors, which is applied to a network system having a NBA (NAT Behavior Aware Server) located in the Internet and connected to two NATs in two private networks respectively. The method enables two network devices in the respective private networks to send testing messages to the NBA via the respective NATs. In response, the NBA sends reply messages to each network device to test the behaviors of the NATs respectively. Afterward, each network device generates a test result message according to each behavior of the corresponding NAT and sends the same to the NBA. Based on the test result messages, the NBA selects an optimal traversal technique from candidate traversal techniques, thereby allowing the network devices to respectively and directly traverse the NATs and establish a direct TCP connection therebetween.