A thermo-fluid simulation was conducted to clarify heat transfer properties and then to show a design window of the first wall of a Li/V blanket with three-surface-multi-coated channels. Channel dimensions were treated as parameters and their effects on the heat transfer properties and the design window were discussed in the various heat flux of 0.8 to 1.2 MW/m2. In the simulation, the magnetic field of 1 and 10T was assumed in order to investigate how the velocity profiles affect the heat transfer performance. The MHD flow was supposed to be two-dimensional-fully-developed laminar flow. Three-dimensional heat-transfer simulation was conducted with assumption of one-side heating by the constant heat flux on the first wall by using the flow field obtained by the 2D flow analysis. It was found that effects of the flow distributions were small on the heat transfer. The channel height, however, affects the heat transfer characteristics so much and the design window is shown to be limited to relatively small channel dimensions due to the induced stress in the first wall.