<正> The effect of involute contact ratio on the torsional vibration behavior of spur gear-pair isstudied analytically through a mass-spring model. The tooth stiffness in model not only has a relationwith time, as many prior studies presented, but, more important, with involute contact ratio (ICR) aswell. The ICR embodies its impact on the spur gear's dynamic performance through changing theproportion of tooth stiffness when there are n+1 teeth in contact to tooth stiffness when there are n