Abstract: A 3H/14C associated sampling device is developed. The 3H and 14C concentration in the air effluent of a heavy water research reactor is measured for 5 times by using it. The results show that the 3H and 14C concentration are 1.29×104 and 220 B q/m3, respectively during the reactor operation period, the 3H concentra tion is mainly affected by the relating reactor core operation, and the  14C concentration being relative to the reactor power rises with the reactor r unning and decreases with the reactor closing.