Heat transfer in a channel of rough wall is simulated in this work. Two kinds of roughness are considered ; i. e., one assumes sand-grain roughness based on roughness model which is just for the profile drag, not implementing thermal effect and the other, regularly arranged two-dimensional ribs which is treated rigourously. For the sake of computational load, one wall is assumed to be smooth and the global Reynolds number Reτ based on global mean friction velocity and half channel width is 150. Mean flow property of velocity and thermal fields are found to be little influenced by the property of roughness elements but depends on total drag, except for in the layer close to the wall where direct interference with roughness elements manifests itself. Similarity of thermal field with velocity one is confirmed for the low Prandtl number flows considered in the work. The mixing is found to be controlled by large scale motion which is inherent to the logarithmic layer, getting closer to the wall.