In real-time applications, such as IP phones and remote conference systems utilizing RTP (real\|time transport protocol) for transmitting continuous media data over Internet, packet loss that impairs quality of transmitting severely is inevitable. It is a better way to solve this problem by adding a function of lost repairing to RTP. In this paper, a method of lost repairing with FEC (forward error correction) based on RTP and an RTP library, which was designed and implemented based on this method are introduced. Then a testing method of lost repairing function and the results are illustrated.