The formation mechanisms and properties of white layers produced in precision hard part machining of GCr15 hardened steels are not clearly understood to date. Microstructure of white layers formed during machining of GCr15 hardened steel (62±2 HRC) was investigated. Hard turned surface with white layer usually consists of three layers: a white layer (untempered martensite), a transition layer (over-tempered martensite), and the bulk material. Martensite, austenite and carbide are observed in the white layer, hardness of white layer is larger than that of bulk material and transition layer, it was caused by extremely small grain size and high dislocation density. The depth of white layer is about 7-14µm with the flank wear reached 0.1mm, the optimal width of the flank wear which generated low depth of white layer is essential to perform the super-finishing process.