Hard turning and grinding are competing precision machining processes for the manufacture of mechanical components such as bearings, gears, cams, etc. Surface hardening at gentle machining conditions has often been reported and is attributed to ultrafine-grained and size effect. However, there are controversial results about surface hardness. Due to the great importance of surface property to component performance such as fatigue and wear, it is imperative to clarify surface hardening mechanisms. The purpose of this paper is to investigate surface hardening and mechanism. Hard turning and grinding of AISI 52100 steel was conducted using gentle machining conditions. Surface integrity was then analyzed in terms of surface microstructure, microhardness, and nanohardness. The research findings showed that the apparent softening measured using microindentation in near surface is not due to thermal effects, but rather a misinterpretation of hardness values due to improper testing technique. Hard turning induces a thicker plastically deformed ultrafine-grained (50∼100 nm) layer than grinding. However, the grinding induced grain size may be smaller that by turning, which produces higher hardness on the ground surface.