The primary chatter vibration of turning tools is investigated experimentally. The vertical displacement of the turning tool and the fluctuating cutting force in the primary chatter vibration are measured simultaneously to investigate the characteristics and the resulting vibration mechanism, when cutting a top of the square thread of mild steel or cast iron. As a result, it is clarified that the relief angle varies widely with the vibration of the turning tool, the primary chatter vibration is self-excited, and the vibratory energy is provided by the frictional force acting in the contact between the tool flank and workpiece.