The analysis of onset and cessation of rainfall for selected locations in South west Nigeria was carried out using daily meteorological data for five synoptic stations (Ikeja, Abeokuta, Ondo, Ibadan and Osogbo) in the region. The FAO Rainfall-Evapotranspiration model was used to elucidate the length of growing season and the impact of climate change on climatic variables and agricultural productivity. Results revealed that the wet season lasts from March to November within the period (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014). The onset and cessation dates varied across the five locations that were considered; Ibadan and Ondo had an early onset of rain while Ondo and Oshogbo had a late cessation, while other stations considered had similar onset and cessation days. The length of the growing season range from 219 -228 days within the region, this suggests the types of crops that can be supported by the rain fed cropping system predominantly practiced in the region and the number of growing cycles that can be accommodated. The study also revealed that weekly analysis of data gave better result than monthly analysis, while daily analysis did not lead to any useful results. The results are of inestimable value for the planning, organization and execution of agricultural activities in the region and in areas of similar climate around the world.