The influences of both the addition of Nb and the heat treatment process on the microstructure and mechanicalproperties of NM550 wear-resistant steel were investigated by means of LM ( Light Microscopy) ,SEM andTEM and by mechanical tests.The results show that ( 1) with the increase of the quenching temperature at a temperingtemperature of 200℃,the grain growth of the Nb-free steel is more obvious with the decreases of the tensilestrength and Brinell hardness as well as with a toughness improvement; ( 2) however,the addition of Nb can effectivelyretard the growth of austenite grain,and the re-dissolution of NbC particles increases the alloying elementconcentration in austenite and decreases the size of NbC particles; ( 3) under the combined action of the solutionstrengthening,the precipitation strengthening and the grain refinement strengthening,the Nb-bearing steel showsbetter mechanical properties; and ( 4) when the tempering temperature increases from 200℃ to 250℃,the mechanicalproperties of the two kinds of steels decrease to varying degrees due to the coarsening of carbide,which indicatesthat the tempering at those temperatures should be avoided.