A one-dimensional steady-state heat transfer model of heat sinks is proposed to determine the heat sink size needed for a given heat source. Both the temperature distribution and the maximum heat source temperature solution are presented analytically. The results are in closed form and can be conveniently used for determining the heat sink size. The two most important dimensionless parameters that describe the geometry and heat transfer characteristics of the heat sinks are defined and their influences on the maximum temperature of heat sinks are analyzed. The results show that the maximum heat source temperature decreases with the heat sink size. It is also shown that if the heat sink size is large enough then the effectiveness in reducing the maximum heat source temperature by increasing the heat sink size is significantly decreased.