Nowadays, the design of radiator of hydraulic system is based on the equilibrium temperature, i.e. under that temperature the heat energy generated by system is equal to the heat energy dissipated. When the working time is not long, the oil temperature can't reach the equilibrium temperature. If the design is still based on the equilibrium temperature, the designed radiator will be larger than needed. This paper studies the temperature rising transient of hydraulic system and suggests a new method of radiator design, which can rationally determine the dissipating area of radiator. This method have been used in the radiator design of a hydraulic crane and get a satisfied result.