This study is concerned with the development of efficient simulation methods for accurate computer representation of synchronous machines in power system transient stability studies;There are a number of models that could be used to estimate the dynamic performance of synchronous machines. In selecting a model in a transient stability study, the trade-off is between accuracy versus computational cost. Detailed examples of the agreements and disagreements between two of these models (simple and complex) are given. A criterion that could be used to determine the proper model (simple or complex) to be used to represent each synchronous machine is developed. The choice is based on fault location, clearing time, parameters, and initial loading of each machine. The study also describes how a combination of models could be used to improve the efficiency of the simulation.