Markets and improved market access are critical for improving urban incomes, particularly in Africa. Despite this, participation of farmers in domestic and regional markets in southwest Nigeria remains low due to a range of constraints. One of the limiting constraints faced by farmers is linked to poor market access. This paper analyses the determinants of commercialization of urban vegetable farming in southwest Nigeria. Primary data were collected for the purpose of this study using structured questionnaire. A multi-stage sampling technique was employed for the study. Oyo and Lagos states were randomly selected from the six states in the southwest Nigeria. The next stage was random selection of two urban Local Government Areas from the selected states. Two hundred and thirty (230) vegetable farmers were randomly selected from both states. The data were analysed using descriptive statistics and Tobit model. Mean age and household size of farmers in the area was 41.9 years and six members respectively. Vegetable farmers are smallholders with average farm size of 0.05ha. Average market participation index of urban vegetable farmers was 0.73. Result of Tobit regression showed that age, age squared, farm size, membership of association, years of education, distance to market, heterogeneity index, cost of fertilizer, cost of chemicals and decision making index influenced extent of commercialization. Social capital significantly affects market participation. The study recommends that governments, non-governmental organizations and other development partners should take a pro-active role in facilitating the formation of smallholder vegetable farmer’s organizations and linking then to markets.