Chemical processes involve various and complicated uncertainties in the design and operation stages. Quantitative analysis of these uncertainties requires a method for stochastic dynamic simulation of chemical processes. In the previous work for the development of a general purpose chemical process simulator capable of stochastic dynamic simulation, only the extent of the uncertainty of a variable was considered. In this work, however, a new parameter MTBD (Mean Time Between Disturbances) is proposed in order to consider the uncertainty on when a change is to occur also. A method for construction of a system which implements the proposed method is also presented. The results of case studies indicate that the proposed methodology can be electively applied to dealing with uncertainties in chemical processes.