Abstract: Using thermochemical treatments, boronized layers were successfully prepared on Al_0.25CoCrFeNi high-entropy alloys (HEAs). The thickness of the boronized layers ranged widely from 20 to 50 μm, depending on the heat treatment time. Boronizing remarkably improved the surface hardness from HV 188 to HV 1265 after treating at 900°C for 9 h. Moreover, boronizing enhanced the yield strength of HEAs from 195 to 265 MPa but deteriorated the tensile ductility. Multiple crackings in the boride layers significantly decreased the plasticity. The insufficient work-hardening capacity essentially facilitated the plastic instability of the boronized HEAs. With decreasing substrate thickness, the fracture modes gradually transformed from dimples to quasi-cleavage and eventually to cleavage.