Laser surface alloying using NiCoCrB alloy (Ni-17.1% Co-19.6% Cr-14.5% Fe-3.5% B-1% C-0.9% Si) on mild steel AISI 1050 and austenitic stainless steel AISI 316L was attempted. Both steels alloyed with NiCoCrB contained austenite as the main phase, with carbides and borides as the minor phases. The maximum hardness of laser-alloyed 1050 and 316L increased to 545 and 410 Hv, respectively, with the cobalt content ranging from 4.5 to 12 wt.%. The cavitation erosion resistance R in deionized water was determined by an ultrasonic vibration system. R was found to increase with the Co content in the alloyed layer, reaching a value 4.4- and 12-fold that of the substrate for 1050 and 316L, respectively. The increase in R was attributed to a decrease in the stacking-fault energy and enhancement of strain-induced martensitic transformability due to the presence of Co. Increase in hardness due to the presence of carbides and borides and a refined microstructure resulting from laser treatment also contributed to an enhanced resistance.