The heat flow characteristics of a high-power optical semiconductor source have been analyzed using a 3D CFD commercial tool, and the thermal resistance values for each of the layers revealed the places for thermal bottlenecks to be improved. As the heat source of a LD (Laser Diode) has a small volume and a narrow surface, the effective thermal cross-sectional area near it is also quite small. It was possible to expand the cross-sectional area effectively by using graphene layers on the TIM (Thermal Interface Material) layers of a LD chip. The effective values of heat resistance for the layers are compared to confirm the improvement effect of the graphene layers before and after, which can be considered to expand the thermal cross section of the heat transfer path.