Abstarct: This numerical study reports on fluid flow and heat transfer characteristics in a cooling channel with various crescent ribs mounted on one wall. The details of turbulent flow structure temperature fields, local heat transfer, normalized heat transfer, and thermal performance factor were obtained using Computation Fluid Dynamics (CFD) with the Reynolds Stress Model (RSM). To validate the numerical model and make the computations convincible, the present numerical results (Nu/Nu0) baxsed on Reynolds Stress Model at the condition of Re=20,000 are compared in region of between 7th and 8th ribs with experimental data. Five kinds of ribs, i.e., the straight rib, the simple crescent rib, the offset mid-truncated crescent ribs, the combined half-size and same-size crescent ribs and the combined half-size and same-size ‘m’ shaped crescent ribs, are considered to improve thermal performance of the cooling channel. The studied Reynolds number varies from 20,000 to 60,000. Mechanisms underlying the enhanced heat transfer by ribs are clarified. It is found that the crescent ribs evidently enhance local heat transfer on the endwall downstream the ribs by generating longitudinal vortices, which intensify flow mixing. Such vortices also increase the turbulent kinetic energy and reduce thickness of the boundary laxyer, which lowers local temperature nearby the target surface. Numerical results show that the cases with crescent ribs significantly outperform the case with straight ribs with respect to heat transfer performance. The channel with the combined half-size and same-size crescent ribs provide a 11.8% higher normalized average Nusselt number relative the straight ribbed channel, and a 7.07% lower pressure drop. Overall, the case with combined half-size and same-size crescent ribs concave to the stream-wise direction provides the best thermal performance.