Abstract: Through experiments investigating the effects of different surface states of quenched and tempered 42CrMo gear steel on gas nitriding, it is concluded that increasing surface roughness within a certain range can enhance nitrided case depth; surface compressive stress reduces case depth and nitrided surface hardness; acid-etched surfaces improve case depth and nitrided surface hardness;Finer and more uniform matrix microstructure with higher matrix hardness in the quenched and tempered condition results in greater case depth and higher nitrided surface hardness.