EVAPOTRANSPIRATION AS AN IMPORTANT FACTOR IN DETERMINING THE WATER REQUIREMENT ARE INFLUENCED BY DIFFERENT CLIMATE PARAMETERS, SO IT’S CRUCIAL TO CONSIDER THE REFERENCE EVAPOTRANSPIRATION AT THE TIME CLIMATE CHANGING. ENSO PHENOMENON IS ONE OF THE FACTORS FOR CLIMATE CHANGING. IN THIS STUDY THE EFFECTS OF ENSO PHENOMENON WAS EVALUATED ON AMOUNT OF REFERENCE EVAPOTRANSPIRATION IN QAZVIN, BY USING THE NINO3.4 INDEX. THE STATISTICAL PERIOD IS 50 YEAR (1957_2006). FIRST OF ALL IT WAS RECOGNIZED THE HOT (EL NINO), COLD (LA NINA), AND NORMAL PHASES OF ENSO PHENOMENON. THEN IT WAS OBTAINED THE CORRELATION BETWEEN REFERENCE EVAPOTRANSPIRATION AND SOME EFFECTIVE PARAMETERS ON THAT, LIKE WIND SPEED, THE DEFICIT OF SATURATION PRESSURE, WITH TEMPERATURE DATA OF NINO3.4 REGION. AVERAGE DISCREPANCY OF OBTAINED TEMPORAL SERIES, WAS EXAMINED AT THE MOUNTS AFFECTED BY ENSO PHENOMENON, BY THE MANN-WHITNEY TEST, IN 99% AND 95% SIGNIFICANT LEVELS. THE AMOUNTS OF REFERENCE EVAPOTRANSPIRATION AVERAGE, ACCOUNTED IN EL NINO AND LA NINA PHASES TO AVERAGE NORMAL PHASE. THE RESULTS INDICATED THAT, THE ENSO PHENOMENON WITH AVERAGE -0.37 CORRELATION COEFFICIENT, WITH REFERENCE EVAPOTRANSPIRATION, CAUSES TO DECREASE 7.8PERCENT OF REFERENCE EVAPOTRANSPIRATION IN EL NINO PHASE TO NORMAL PHASE. AND ALSO REFERENCE EVAPOTRANSPIRATION IN EL NINO PHASE TO LA NINA PHASE DECREASE ABOUT 10.6 PERCENT. THE REFERENCE EVAPOTRANSPIRATION IN LA NINA PHASE HAS NOT SIGNIFICANT DIFFERENCE WITH NORMAL PHASE.