Recently safety systems for commercial vehicles have been developed rapidly. However, there are still many problems in the vehicle stability and the braking performance. In particular, a commercial vehicle may meet a dangerous braking condition when the vehicle is lightly loaded or empty and when the road is wet or slippery. Under these conditions, the truck can spin out, the tractor can jackknife or the trailer can swing out. To design an air brake system for a commercial vehicle, since the air brake system has many design variables, there must be intense research on a method on how to prevent dynamic instability and how to maximize vehicle deceleration. In this study, mathematical models of the tractor-semitrailer and the air brake system including an antilock brake system controller have been constructed for computer simulation. Also, simcopy IMple examples are applied to show the usefulness of the program. Designers can use this simulation program for understanding the braking characteristics such as trajectory, braking distance, longitudinal deceleration, lateral deceleration and yaw rate under various braking conditions.