The stabilizing threshold of chatter vibration is obtained from the characteristics of resonance curve in machine tool system, which can be established the dynamical amplification coefficient and spring constant. This study is taken the external cutting of a round bar by right handy blade bit with side cutting edge angle. The bit is fixed tightly in the tool post of lathe. A round bar fitted between the both center is treated as bending vibration. In lathe turning under the vibratory characteristics, the cutting area change between normal cutting and outbreak of chatter vibration after one rotation of work becomes to alter by the size of bit and cutting condition. When the cutting area change is set theoretically, the machine tool characteristics become to influence chatter vibration in lathe turning. In this study, the difference of specific cutting resistance before and after the chatter, vibration in normal force direction is taken by− 0.10 kgf/mm2, 0 kgf/mm2, : 0.10 kgf/mm2, 0.30 kgf/mm2, and 0.50 kgf/mm2 . And dynamical amplification coefficient in machine tool system is analyzed from the formula of stabilizing threshold.