The effect of involute contact ratio on the torsional vibration behavior of spur gear-pair is studied analytically through a mass-spring model. The tooth stiffness in model not only has a relation with time, as many prior studies presented, but, more important, with involute contact ratio (ICR) as well. The ICR embodies its impact on the spur gears dynamic performance through changing the proportion of tooth stiffness when there are n+l teeth in contact to tooth stiffness when there are n teeth in contact. A couple of curves about impact of ICR on the gears dynamic performance are presented, and they clearly demonstrate that the model can accurately describe the effects of ICR on dynamic transmission error. Finally, some conclusions useful to reduce vibration and noise of gear-pair are proposed.