Ratcheting is a cyclic accumulation of inelastic strain occurred under the stress--controlled cyclic loading with< non--zero mean stress, it is very important in assessing the fatigue life and safety of the structure subjected to cyclic loading. The existed results showed that different materials presented different ratcheting features, and the results were obtained from the cyclic tests with a small number of cycles (less than 200 cyc). Therefore, based on the uniaxial strain--controlled and stress--controlled cyclic tests with high number of cycles (more than 2000 cyc), the cyclic deformation features of 20 steel were observed at room temperature. The effects of the cyclic softening/hardening feature and yielding plateau of the steel and the applied mean stress, stress amplitude and stress ratio on the ratcheting of the steel with high number of cycles were studied. The results show that the 20 steel presents a weak and stain--amplitude--dependent cyclic hardening, and the ratcheting of the steel depends greatly on the applied mean stress, stress amplitude and stress ratio. The ratcheting strain increases monotonically with the increase of applied mean stress and stress amplitude, but decreases with the increase of stress ratio. A re--speeded ratcheting will occur at the end of cycling with high number of cycles when the applied stress levels are high enough. More importantly, the yielding plateau of the steel greatly influences the evolution of the ratcheting. It is concluded that the effect of yielding plateau on the ratcheting should be reasonably considered in constructing the cyclic constitutive model of the ratcheting for the 20 steel.