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The aim of present study is to investigate the kinematics of tool-workpiece’s relative movement in conventional and ultrasonic-vibration assisted turning (UAT). The kinematic analysis of UAT shows that the movement of cutting tool edge relative to the workpiece resulted from the cutting speed, feed speed and tool’s vibration affects the lateral machined surface of workepiece and leaves a repeating pattern of crushed and toothed regions on it. This results in an increase in the surface hardness of the lateral machined surface in comparison with conventional turning (CT). A model of the tool-workpiece’s relative movement has first been developed in the present study. This model predicts a surface hardening effect for the lateral surface in UAT in comparison with CT. Several experiments were subsequently carried out employing a surface micro-hardness testing machine and an optical microscope to verify the predicted results.