Heat transfer in a rotating rectangular duct was numerically simulated by using the fourth order finite difference method. In order to investigate the effects of the Coriolis force and the duct cross-sectional aspect ratio on the turbulence, the large eddy simulation was adopted with a dynamic subgrid scale model. In the computation, the aspect ratio and the rotation number were varied from 0.25 to 4.0 and from 0 to 5.0, respectively, while the turbulent Reynolds number was set to be a constant value, 350. In this aspect ratio range, both direct and indirect effects of the Coriolis force on the turbulence were seen, that is, the direct influence on the fluctuating velocity and the indirect influence through the modified streamwise velocity profile by the secondary flow. The relative degree of these influences depended on the aspect ratio. The Colburn's j factor showed the higher value for the larger aspect ratio cases because of the larger heat transfer enhancement on the pressure and side walls.