Abstract: This paper addresses the problem of optimal design of batch processes with uncertain processing times.A method is proposed for defining the limiting cycle times under uncertain processing times.The limiting cycle times represented in the model become independent of each other.Thus,the process can be transferred into a renewal process and a stochastic optimization formulation is developed based on renewal process theory.An example shows that the optimal design considering the uncertainty of processing times can give better results than that without considering the uncertainty.The method is shown to be effective by simulation experiments.