Abstract: By studying the construction of the RTT latency, alinear expression between the RTT latency and the packet size is presented. Then pathchar,traditional bandwidth measurement algorithm,based on the relationship between the RTT latency and the packet size, is analyzed particularly. But pathchar can not measure the asymmetric link bandwidth. To solve this problem, Asy_pathchar, a new algorithm, is put forward, which can measure each hop's link bandwidth in both directions in an IP network using the UDP packet and ICMP packet.Finally an experiment is given to validate the new algorithm.