CrMnFeCoNi high entropy alloys show high strength and ductility at low temperatures because of twinning-induced plasticity (TWIP) due to low stacking fault energy. The addition of Si to CrFeCoNi alloys is expected to improve its strength because locally increasing the stacking fault energy is predicted by first-principles calculations. For verification of the prediction, CrFeCoNi-Si alloys were prepared and the mechanical properties were evaluated. The alloy samples with the maximum additive amount of 5 at % were prepared by MoSi2 electric furnace. Vickers hardness measurement and tensile test of the samples were carried out. As a result, Vickers hardness increased with increasing Si content. The yield strength and the tensile strength of a Si free alloy at liquid nitrogen temperature showed 390 MPa and 700 MPa, respectively, and those with Si addition of 5 at% were 435 MPa and 808 MPa, respectively, indicating that the yield strength and tensile strength improved with increasing Si content.