We mainly explore two problems when combining IPSec module into TCP/IP stack by porting the famous IPSec software (FreeS/WAN) into a security gateway. One is how to implement the IPSec module based on Netfilter in Linux 2.4. x kernel. The other problem is the performance evaluation. We test the throughput of our security gateway before and after applying IPSec with different encryption/decryption algorithms, including the software-based and hardware-based method. With these testing data, we analyze further system performance bottleneck. In the end, we also infer the quantitative relation between the system throughput and the speed of encryption/decryption algorithm and propose some valuable conclusions for improving performance.