ABSTRACT: Finding out The Probable Maximum Precipitation (P.M.P) and probable maximum flood are among the important issues of designing hydrologic structures. One of the most suitable techniques for predicting probable maximum precipitation is the envelop curve method. According to this method all records of m precipitation are plotted in a log-log graph and finally a line which covers all events of precipitation is drawn. The results gained from such methods provide higher accuracy as well as reduces the cost of projects which is a very important issue in decision making. Accordingly envelop curve of precipitation was prepared in current research and the results showed that maximum 24-hour precipitation for Semnan province is 42 mm while the results for 48 hour, one month and one year data were 61, 135.2 and 344.2 mm respectively.