The dynamic response of a single pair of gears was analyzed in consideration of the time varying mesh stiffness, gear errors, transmission errors. The dynamic response model was set up by using torsional vibration of the physical model. The time varying mesh stiffness is calculated by substituting the variance of the contact line length, when the unit contact line length of the mesh stiffness is constant. Then the node displacement of the gear was solved by using the Runge-Kutta method. The influence of geometry parameters of the gear on the vibration and noise of gear meshing was analyzed, and the noise due to vibration is quantitative calculated by using Kato method. It is shown that this method is a good ideal method for quantitative calculating the noise due to gear vibration. It gives examples to provide basis for low noise design and noise - decreasing of gear system.