Abstract: Radiation heat transfer between fins and tube wall of the radiator of straight fins-tube was calculated by Monte Carlo Method.The impact of quantity of fin,fin height,fin thickness and the temperature of tube wall on heat rejection performance of the radiator under the first boundary condition were analyzed by numerical simulation.The analysis shows that,under a certain fin mass,there is an optimum quantity of fin and fin height to get the highest heat rejection efficiency.Fixed the fin shape,adding the fin thickness is not an effective way to increase the efficiency.The heat rejection efficiency of the fin is decreased with the temperature of tube wall decreasing,and the necessity of fitting fin is reduced when the wall temperature is higher.