The enhancement of heat transfer in cooling system of cylindrical lithium-ion battery pack is numerically investigated by installing fins on the cooling plate. Battery Design StudioⓇ software is used for modeling electro-chemical heat generation in the battery and the conjugated heat transfer is analyzed with the commercial package STAR-CCM+. The result shows that installing fins on the cooling plate increases the convective heat transfer on the surface and thus lowers the maximum temperature of the battery pack. As the length and thickness of the fins increase, heat transfer in the battery pack improves. Considering the geometry and airflow of the battery pack, the optimal values for the length and thickness of the fin are both 2mm. As the convective heat transfer coefficient of the surface increases, the maximum temperature of the battery pack is greatly reduced and the temperature gradient is greatly improved.