A heat sink is a kind of heat exchanger that cools a device by dissipating heat into the environment. The design of heat sink by numerical simulation depends to a large extent upon boundary condition between the heat sink and its surroundings. The boundary condition is usually defined by Heat Transfer Coefficient (HTC) which can be used to measure heat resistance between heat sink and surrounding. Determination of HTC is a difficult problem, depending on many factors such as air velocity, material properties, shape of radiators and surface treatment, etc. A new simple reverse method has been suggested in the present work. The reverse method is suggested according to Mean-Value Theorem in mathematics. By comparing the temperature curves between simulation and real measurement, the temperature differences can be minimized by selecting different HTCs.