This experiment was conducted to compare the effects and bioavailability of DL-methionine (DL-Met) and L-methionine (L-Met) supplementation in maize-soybean meal-based diets on performance and carcass traits of broiler chickens. A total of 728-day-old male broilers were divided into 7 groups with 9 replicates (7 replicates of 12, and 2 replicates of 10 chicks) using a 2 × 3 + 1 factorial arrangement in a completely randomised design. Dietary treatments consisted of basal diets (BD) (including 0.619, 0.555 and 0.523% digestible methionine + cysteine (Met + Cys) for starter, grower and finisher periods, respectively) supplemented with three levels (0.155, 0.310 and 0.455%) of either DL-Met or L-Met: BD, BD + 0.155% DL-Met, BD + 0.310% DL-Met, BD + 0.455% DL-Met, BD + 0.155% L-Met, BD + 0.310% L-Met and BD + 0.455% L-Met. The interaction between sources and levels of Met supplementation did not influence overall growth performance, yield of carcass and parts, as well as relative internal organ and feather weight. Source of methionine had no significant effect on overall growth performance, yield of carcass and parts, relative internal organ and feather weight, while methionine addition significantly improved growth performance and carcass and cut yields. The slope-ratio assay showed that the relative bioavailability (RBV) of L-methionine to DL-methionine for BW, FCR and BMY of broilers were 123, 91.5 and 88.0%, respectively, i.e. the differences were not significant between the two Met sources. In conclusion, our data indicated that when DL-Met and L-Met were included in feeds at practical levels, they were equally effective as a source of methionine for broilers.