Abstract: Although re-engineering is often used as a means to reduce the risk, reduce costs and maintain the operation of deriving the software, but re-engineering cannot avoid risk. Risk is a probability of occurrence of some unwanted and harmful event to the project. These events can result in cost overruns, schedule slippage, or failure to meet their project goals. Simulation is an important technique for calculate the probability of risk in software reengineering projects. This paper is aimed at implementing a simulator in Monte Carlo algorithm and Matlab language which estimates the probability of risk during project completion.