Abstract: The time-varying meshing stiffness is one of inherent characteristics of gear system vibration.The meshing stiffness of an external and inner helical gear pair with different helical angles was solved respectively using a modified method,which could determine the meshing stiffness and actual load distribution based on linear programming.The variation rules of meshing stiffness during one whole meshing period were summarized.The following findings were obtained in this paper: the key factor that influences the meshing stiffness most significantly is the whole length of contact-line of all the teeth in the engagement.The meshing stiffness decreases at the instantaneous position where the teeth enter contact or exit contact.The results were compared with those obtained by aerospace industry ministry standard(HB/Z 84.1-1984) and the influence of rim thickness on the meshing stiffness was analyzed.