Reference evapotranspiration (ETo) is an important agro-meteorological parameter for climatological and hydrological studies, as well as for irrigation planning and management. It is a climatic parameter and can be computed from weather data. This work evaluated the correlation of ETo with climatic parameters for Ibadan, Sokoto and Kano; in order to assess their effect on ETo since the rate of ETo depend on the available climatic parameters. The climatic data used were obtained from International Institute of Tropical Agriculture, Ibadan (1977-2010). The ETo was estimated using the FAO-56 Penman-Monteith Method. The results show that solar radiation, sunshine hour, air temperature, wind speed and clearness index were positively correlated with ETo while relative humidity and cloudiness were negatively correlated. The correlation coefficients obtained ranged from 0.538 – 0.871, 0.164 – 0.953 and 0.031 – 0.994 for Ibadan, Sokoto and Kano respectively. The result also shows that the correlation of each parameters with ETo depend on the pravailing climatic condition of the location. This work has shown the degree of the relationship between ETo and climatic parameter, which depend on the prevailing climatic conditions of the study area. The results obtained can help agriculturists and engineers for effective irrigation planning and management. The results obtained were compared with that of other researchers.