In an optical module with high density and small size, it is important to maintain the operating temperature by adequate heat dissipation. The thermal simulation of an optical module for communication has been performed to reduce the operating temperature. To improve the behavior of heat dissipation, we have simulated heat path and heat resistance using commercial code, COMSOL. We have chosen simulation parameters, which include thickness, thermal conductivity, interface area and surface roughness of heat sink and submount as an element of heat resistance. To reduce heat resistance, it was desirable to use the material of higher thermal conductivity, single crystalline silver, and to control the surface roughness of the interfaces. We could lower the operating temperature of an optical module about 18 degrees by controlling the above parameters.