Since the introduction of CoCrMo alloy metal-on-metal hip replacements have shown a great clinical performance. Metal-on-metal couplings produce a much lower wear rate and volume than e.g. metal-on-polyethylene. However, the particle size is significantly smaller within a nm-range. To evaluate the formation of nano-size wear particles in metal-on-metal hip replacements it is essential to understand the micro-structural changes in the sub-surface region of the CoCrMo alloy. For this study a MoM hip implant was analyzed by means of TEM. The results revealed that the good wear performance of this CoCrMo alloy is linked to a strain induced fcc hcp phase transformation and in-situ re-crystallization under high shear stresses. The result is a nano-crystalline surface zone of ~200 to 400 nm thickness which undergoes an ongoing process of mechanical intermixing with componants of the interfacial fluid. The incorporation of organic carbon from proteins in between the nano-crystals could be visualised by EFTEM and EDS. This mechanically mixed nc-zone must be the origin of the wear particle detachment. An earlier study by Catelas et. al confirms the hypothesis of the location of wear particle detachment by analyzing the shape and chemical composition of emitted wear particles which exhibits the same size and shape of crystals observed in the nc-zone of the implant analyzed in this study.