Abstract: An aircraft servo loop is designed by using the variable structure system （VSS） theory. The detail design procedure and a simulation block diagram are provided. Then digital simulation results which was done on IBM4341 computer show that some outstanding advantages of VSS comparing with general control systems are as follows: （ 1 ） a rapid transient response, no overshoot, no steady error. （ 2 ） more robust to the varience of system parameters. （ 3 ） more robust to the outside disturoences.