This paper deals with a computer simulation for the stochastic inventory system in which the decision rules are associated with the problem of forecasting uncertain demand, lead time, and amount of shortages. The model consists of mainly three parts; part I$cdots$the model calculates the expected demand during lead time through the built-in subrou tine program for random number generator and the probability distribution of the demand, part II$cdots$the model calculates all the possible expected shortages per lead time period, part III$cdots$finally the model calculates all the possible total inventory cost over the simulation period. These total inventory costs are compared for searching the optimal inventory cost with the best ordering quantity and reorder point. An application example of the simulation program is given.