Roll over valve has strict requirements about tightness when working under micro pressure.The factors influencing on the leakage of roll over valve were analyzed by theoretical analysis method. A testing method of leakage under micro pressure was presented, and the testing system based on this method was developed.In this test system, precision regulator, pressure gauge and other components were used to control and test the related parameters, and a software control system based on Visual C++ and PLC was included. Experimental results show this test system can test the tightness performance of roll over valve efficiently. Theoretical analysis and experimental results show that testing pressure, balance time and test chamber volume have direct impacts on leakage.