Abstract: In order to investigate the failure mechanism of medium bainitic steel backup roller under rolling contact fatigue, the uniaxial-strain cyclic characteristics and ratcheting behaviors of the material were studied by experiments at room temperature. The cyclic softening / hardening feature of the material and its effect on the uniaxial ratcheting were discussed. Simultaneously, the dependence of the ratcheting on mean stress, stress amplitude and loading history was also revealed. It is shown that: the cyclic softening/hardening feature of the material presented remarkable strain-amplitude dependence, and different ratcheting behaviors were caused in the uniaxial cyclic stressing with different stress levels. Some significant conclusions are obtained, which are useful to construct a constitutive model to describe the uniaxial cyclic deformation of bainitic steel.