Abstract: The object of this study was to compare seed yield and yield related characters of cowpea cvs. Karagoz-86 and Akkiz-86 under both rainfed (non-irrigated) and irrigated (well-watered) conditions in Samsun, Turkey in the years of 2005 and 2006 and also to determinate reduction in seed yield under rainfed condition. The field experiments were arranged in Completely Randomized Design with ten replications. Significant differences (p<0.01) were found among study years for plant height, 100 seed weight, the number of seeds per plant, seed yield and biological yield. Cultivars differed for (p<0.01) 100 seeds weight, the number of seeds per plant, seed yield, biological yield and harvest index. Karagoz-86 gave higher seed yield (116.36 g m-2) than Akkiz-86 (53.51 g m-2) due to it has larger seeds and more seeds per plant. Except for the number of branches per plant, 100 seed weight and harvest index, plants grown in irrigated conditions gave higher means regarding all characters when compared with plants grown in rainfed conditions. Seed yield reductions of 64.01 and 43.11% were determined in Karagoz-86 under rainfed conditions when compared with irrigated conditions in 2005 and 2006, respectively. Yields of Akkiz-86 under rainfed conditions were lower 58.02 and 31.50% than that in irrigated conditions for the same years.