The performance evaluation is an important aspect in analysis and evaluation of workflow models. In this paper, the general Petri net models for business proced ures are introduced first and then the extended time model of workflow nets is e stablished. On the basis of reachability graph, the concepts of simple routing g raph and transformable subnets are advanced. By using net transformation methods keeping response time and assigning probability, the extended timed workflow ne ts are simplified. The algorithm for searching transformable subnets and the met hod for calculating time performance of workflow models are provided.