Evapotranspiration (ET) is a major component of hydrological cycle and the rate of ETo is mostly affected by climatic parameters. This research work evaluates the seasonal and annual variations of ETo with climatic parameters in Ibadan from 1977 - 2010. The climatic data used were obtained from International Institute of Tropical Agriculture, Ibadan. FAO-56 Penman-Monteith method was used in estimating ETo. The results show that the monthly mean ETo varies in the same pattern with solar radiation, air temperature, sunshine hour, evaporation and slightly with wind speed, but varies in opposite direction with relative humidity. Hence, low ETo were obtained in rainy season when it was cool, cloudy and humid while high ETo were obtained when it was hot, sunny and dry. Annually, the gradient of variation of ETo with evaporation, solar radiation, wind speed, and relative humidity in the area within the period under study is positive, implying an increase in these parameters, while ETo have negative gradient with sunshine hour and temperature. This variation may be due to the location of Ibadan in the tropical rain forest. The results obtained can help agriculturists and engineers for effective irrigation planning and management.