The effects of the prior austenite grain size in deep cryogenic treatment on the hardness,the structural change and the wear resistance of D6 tool steel were investigated.The wear resistance of the cryogenically treated samples was determined using the pin-on-disk wear machine.The microstructural characteristics and phases present in heat treated samples were determined using SEM and XRD techniques.The results showed that the retained austenite is completely transformed to martensite during the cryogenic treatment.Besides,there is an optimum grain size of which the maximum wear resistance and hardness are obtained. The effects of the prior austenite grain size in deep cryogenic treatment on the hardness, the structural change and the wear resistance of D6 tool steel were investigated. The wear resistance of the cryogenically treated samples was determined using the pin-on-disk wear machine. The microstructural characteristics and phases present in heat treated samples were determined using SEM and XRD techniques. The results showed that the retained austenite is completely transformed to martensite during the cryogenic treatment. Besides, there is an optimum grain size of which the maximum wear resistance and hardness are obtained.