Transmission line matrix (TLM) method is a very effective time domain method, which has been used for solving various electromagnetic problems including waveguide structures. In this study, the transient response of a rectangular waveguide is analyzed by using TLM algorithm which is also very suitable for the parallel computing. The problem is extended for parallel environment for the speed up and scalability analysis. Excellent agreement with analytical solution is reported. (C) 2007 Wiley Periodicals, Inc.