The paper reports numerical results obtained in a fully developed turbulent channel flow with triangular riblets. The heat and momentum transfer characteristics are calculated by using a nonlinear low-Reynolds number k-epsilon model together with several turbulent scalar flux representations. Comparison of the drag variation prediction with previous experimental and numerical data shows that the present turbulence model can simulate the turbulent flow over this complex surface reasonably well. By comparing the results obtained with the isotropic and anisotropic Reynolds stress representations, the significance of the mean secondary flow in the drag reduction mechanism is discussed. In both drag reducing and increasing conditions, the drag change predicted is found to be in close agreement with the data available in the literature. Under the drag reducing conditions, possible departure from the Reynolds analogy is investigated at various Reynolds and Prandtl numbers. It is shown that, while the skin friction is reduced over the riblet surface, the heat transfer performance can actually be increased beyond that on a smooth wall, although discrepancy with the experimental data is not negligible.