This study investigated the effects of concentrate levels and oil supplementation on rumen characteristics and growth performance of cattle. Twenty-four Brahman crossbred bulls were arranged in a 2x3 factorial design with two levels of concentrate (0.5 and 1.5% body weight) and three supplemental sources (60 g soybean oil/kg dry matter (DM), 60 g fish oil/kg DM and non-oil). Total rumen bacteria number was higher in cattle fed diets with lower concentrate level (P