A numerical study of three-dimensional mixed convection for a constant-property fluid flowing laminarly through a periodical two-pass square channel with radial rotation is presented. Thermal and fluid-flow fields are calculated for the entire domain from the Navier-Stokes equations and the energy equation by a finite-difference technique. The emphasis is placed on the rotating effects, including both the Coriolis force, and centrifugal buoyancy, on heat transfer in channels of different flow directions. Typical developments of the axial velocity, secondary flow, and fluid temperature are presented to illustrate in detail the influence of rotation on the heat transfer. The results reveal that, in a buoyancy-affected rotating two-pass channel, the local heat transfer is dependent on the dow direction, and the Coriolis-force effect is more notable for the radially inward dow than that for the radially outward how. It is further found that the centrifugal buoyancy enhances significantly the peripherally averaged heat transfer in the radially inward-flow channel, but relatively negligibly in the radially outward-flow channel, which is analogous to the buoyancy-dependence of the cross-dow-intensity development. The prediction further demonstrates that, as long as the centrifugal buoyancy is sufficiently strong, the radially outward flow will separate from the leading surface, which largely deteriorates the wall heat transfer. A comparison of the present prediction with available numerical and experimental data for stationary and rotating ducts is also presented.