Since the frequency of rainy days is one of the decisive factors in determining the climatic conditions of a land, the issue of determining the seasonal share of the annual frequency of precipitation days was considered in a 31-year interval (1973-2003) in the western zone of Iran enjoying high rates of precipitation (in this research including Zanjan, Qazvin, Markazi, Lorestan, Kordestan, Ilam, Kermanshah and Hamedan), in order to help better utilization of valuable soil and water resources. This important task was accomplished by collecting the frequency figures of monthly rainy days from 16 meteorological  stations in the above mentioned provinces. Comparison of mean values ​​between seasons and year of land zoning based on the coefficients determined by regressive equations showed that winter season, in comparison with other seasons in most regions, has the highest coordination and also the highest spatial  equilibrium in terms of precipitation amounts and number of precipitation days. . This finding may indicate an increase in the frequency of precipitation days in other seasons in the years when winter has less rainy days. Only 5 in cities, including Khorramabad, spring has replaced winter as the reliable  season in terms of precipitation.