To better evaluate the application of ribs in gas turbine blade cooling, this numerical study reports on the effects of ribs mounted on the internal cooling channel wall on external adiabatic film cooling performance. The numerical model is validated against available experimental data. For fixed mainstream and cooling flow Reynolds numbers, three ribs including the continuous rib, centrally truncated rib and laterally truncated rib at two blowing ratios (i.e., 0.5 and 1.0) are considered. At lower blowing ratio, results show that the cases with three distinct ribs provide approximately identical cooling effectiveness but significantly outperform the case without ribs. The superiority regarding laterally averaged adiabatic cooling effectiveness is 17-157%. However, at higher blowing ratio, the cases with ribs are inferior to the case without ribs, showing an inferiority of 21-68%. Further, at higher blowing ratio, the case with continuous ribs exhibits a 12-45% higher localized cooling effectiveness relative to other ribbed cases. It is found that cooling effectiveness deteriorates with the increase of spiral intensity of the coolant flow inside the film hole. Lower spiral intensity leads to a better covering of target wall by the coolant and thus results in better cooling effectiveness. The distinct relative merits among the cases at different blowing ratios are explored based on the aforementioned dominant mechanism. (C) 2016 Elsevier Ltd. All rights reserved.