In the present work, the sliding wear behaviour of surface mechanical attrition (SMA) treated AISI 304 stainless steel by spherical shot peening has been studied under both unlubricated and lubricated conditions under various contact loads. It was found that although SMA treatments could produce a hardened layer of several hundred microns thick with much increased hardness from 200 HV0.1 to 480 HV0.1, SMA treatments did not have a significant effect on the unlubricated wear behaviour of the steel. However, under oil-lubricating conditions, the SMA treated steel showed much better wear resistance than the untreated steel over a wide range of contact loads. The results are discussed in terms of plastic deformation and property changes induced during the sliding wear process. ► SMAT produced a 300μm thick hardened layer with much increased hardness. ► SMAT did not significantly affect the unlubricated sliding wear behaviour. ► SMAT is effective in enhancing wear resistance under lubrication condition.