In this study، trends of threshold heavy precipitation were examined on the basis of measurements of 1437 stations (included synoptic، climatic and rain gage stations) in Iran for the period of 40 years (1964-2003). For this purpose، the statistical method of nonparametric Mann-Kendall test is used for analyzinging the thresholds of heavy precipitation and nonparametric Sen Method for estimating the slope of a linear trend. The results showed that there is a significant increasing trend in threshold of annual heavy precipitation over Iran، especially from 1992 to 2003 and this increasing is estimated as an average of 0.5 mm per year. Meanwhile، maximum and minimum thresholds of heavy precipitation which is estimated are obtained respectively 0.78 mm and 0.22 mm for every year.