This paper addresses farmers' participation in On-Farm Adaptive Research (OFAR) in South-western Nigeria. The concept of OFAR entails full participation of farmers in the research process, direct contact between researchers and farmers and intensive investigation of farmers' situations to strengthen Research-Extension-Farmers Linkage. Farmers' level of participation is critical in creating room for consideration of local ethics, culture, environmental and socio-economic characteristics to enhance successful and accepted programmes. This study sought to assess the farmers' level of participation based on four major phases of OFAR process: diagnostic survey phase, research phase, field test phase and demonstration phase. A multi-stage sampling technique was employed to collect cross sectional data from 350 farmers in Southwestern Nigeria. Farmers were favourably disposed to participation and inadequate input, capital, access to information; time and non-availability of market were major constraints against participation in OFAR. The implications of these findings for both research professionals and farmers are discussed.