In this study, the interpolated data among Iran weather stations were used to study the PRECIPITATION REGIME variations by harmonic analysis during recent four decades (1965-2004). After verification of the data with statistical tests during the period of study, it was found that there are no significant variations in the variance of the first harmonic. This result highlights that the dominant PRECIPITATION REGIME in Iran is caused by winter season. The magnitude of the variance of the second harmonic is accompanied by variations and displacements of iso-hyet lines. Evaluation of the third and forth decades illustrates a remarkable increase in the variance of the third harmonic. This indicates that local PRECIPITATION REGIMEs will be expected in some regions of Iran. In Azrarbaijan (North-west), the amplitudes of the second harmonic decline from the first decade and reach the minimum value as we move toward the forth decade. This emphasizes that the total springTIME convective PRECIPITATION has reduced in the aforementioned region. During the third and forth decades, the eastern, southeastern and southwestern regions of Iran and Zagros highlands show higher third maximum harmonic, in comparison with the first and second decades. The mentioned increase in PRECIPITATION variance was higher in Sistan and Baluchestan than in other regions. This result confirms the occurrence of significant variations in PRECIPITATION REGIME in this part of the country.