١ Simulation Approach to Estimate the Activity's Expected Running time Abstract Activity time is the elapsed time required for an activity. Estimating is an inexact art, so we expect that our initial duration estimates have some error in them. What we would really like to know is how much this error is going to affect our estimate of the total project duration. Fortunately, with a few assumptions and very little extra work we can make some judgments about the likely amount of variation in the total project time. Estimating activity times is probably one of the project Evaluation and Review Technique's (PERT) most critical features. The concept of three times estimating are used(Optimistic Time, Most Likely Time, and Pessimistic Time). However, the best estimate of the time period (Most Likely Time) in which the activity can be accomplished was provided by experience. In this paper we attempt to optimize the expected running time estimates and the most accurate estimates of the activity performance time using simulation . New method was proposed for this purpose and some details was explained about this method. The result show that the simulation is necessary to optimize time as well as it's flexibility. 1. Introduction The Program (or Project) Evaluation and Review Technique, commonly abbreviated PERT, is a model for project management designed to analyze and represent the tasks involved in completing a given project. It is commonly used in conjunction with the shortest and critical path method or CPM. It is a method to analyze the involved tasks in completing a given project, especially the time needed to complete each task, and identifying the minimum time needed to complete the total project. Historically , PERT was developed primarily to simplify the planning and scheduling of large and complex projects. It was developed for the U.S. Navy Special Projects Office to support the U.S. Navy's Polaris nuclear submarine project, [Malcolm, D. G et al. , 1959]. It was able to incorporate uncertainty by making it possible to schedule a project while not knowing precisely the details and durations of all the activities. It is more of an event-oriented technique rather than start- and completion-oriented, and is used more in projects where time, rather than cost, is the major factor. It is applied to very large-scale, one-time, complex, non-routine infrastructure and Research and Development projects [Winter Olympics official report, 2010]. This paper focuses on simulation to estimate the best activity performance time . Rest of the paper is organized as follows. section 2 present definition of PERT with some details. In section 3 our proposed method is described in detail step by step . Muhsin Hasan Hussein Karbala University College of science\computer Departement Mohsin.alqassab@yahoo.com Dr. Saad Talib Hasson College of science\computer Departement