Abstract: In this paper, Al_xCoCrFeNi alloys were prepared by vacuum arc melting technology, and the effect of Al content on the microstructure and mechanical properties was studied. In addition, the mechanical properties and strengthening mechanism of the alloys with different Al contents were also systematically explained. The result showed that the surface morphology and phase composition of the alloys could be significantly changed as Al content increased, and their mechanical properties and strengthening mechanism could also be substantially affected. When x = 0.10 and x = 0.30, the strength and plasticity of the alloys increased obviously compared with that when x=0. The primary strengthening mechanism was solution strengthening and grain refinement strengthening, and the increase in their plasticity was mainly due to the plastic deformation induced by the twinning, which could be related to the reduction in stacking fault energy as the alloying elements increased. When x=0.50 and x = 0.75, the strength of the alloys increased obviously, but their plasticity decreased significantly. With increased Al content, the solution strengthening of the alloys and the content of the body-centered cubic (bcc) phase increased, and the grain size decreased, all contributing to the higher strength. However, when x=1.00, the strength of the alloys decreased significantly and there was almost no plasticity. This was because the alloy was composed entirely of the bcc phase with extremely small grain size, making the sample brittle. Consequently, it fractured and failed before reaching its elastic limit, resulting in no apparent yield phenomenon and practically no plastic deformation.