Hard to cut materials such as hardened steel and super alloys are well known and popular in transportation, aerospace, marine and medical industries, due to their strength, high corrosion and fatigue resistance and high working temperature . However, their lowmachinability is one of the major problems in using them. In this article we examine machinability of the hardened CK45 steel in four processes including; conventional turning, hot turning, ultrasonic assisted turning and hot ultrasonic assisted turning as a hybrid machining. The results of these four processes are compared with each other. The heating technique is induction type which has much lower construction cost compared to other techniques such as laser beam and plasma arc. It has been illustrated in the present study that combining the heating technique with ultrasonic vibration has the most effect in improving surface quality and reducing cutting forces compared to other turning process. The chip formation under the heat and ultrasonic vibration and its influence on the surface quality have also shown that hybrid process has the most affection.