Reference evapotranspiration is very important because it correlates with the amount of water required by crops and also plays very key role in the hydrological cycle. Evaporation is the process of water loss from the earth surface in which temperature effect is significant while transpiration is water loss from plants. Studying evapotranspiration is also important because of the link between climate change and water scarcity. The reference evapotranspiration for Gassol was estimated and analysis done to observe its trend and variation. In this paper, the FAO Penmann-Monteith model was used to estimate the reference evapotranspiration for Gassol town located in the Sudan Savannah vegetation belt of Nigeria. The annual monthly estimates show a generally recurring seasonal pattern of variation from 1985 to 1991. January through June had lower ET0 compared to July through December. The time series plot of the ET0 estimates from 1985 to 1991 in monthly renditions gives a cyclical pattern of variability with most of the years showing bimodal peaks. Also, an evenly spread data was presented by the normal distribution curve. The periodogram of the estimated reference evapotranspiration gave a dominant periodicity of 9.33 months cycle. The estimates of and the pattern of variation of the reference evapotranspiration as observed for Gassol in this study will very likely experience a continuous downward trend. For proper irrigation management, January to March and October to December should be properly planned.