Uniaxial ratchetting characteristics of 20MnMoNi55 steel at room temperature were studied experimentally. The effects of stress amplitude and mean stress on the ratchetting behavior of the steel were analyzed. It can be concluded that the ratchetting strain amplitude and ratchetting strain rate of 20MnMoNi55 steel increases with increasing stress amplitude or mean stress correspondingly. The steel exhibited large accumulation of ratchetting strain which is attributed to its cyclic softening characteristic. Variation of ratchetting strain and strain rate with cycle number displayed three stage behavior.