The recoil-distance Doppler-shift (RDDS) methods is an accurate and direct method to measure lifetime of nuclear levels in 10-12-10-9s range. An apparatus based on the RDDS method was designed and tested. An in-beam experiment with the apparatus was first performed in China. Lifetimes were measured for the levels of 120Xe ground band and a few levels of 121Cs and 117I which are produced by bombardment on 93Nb target with 31p beam.