An approximate equation has been proposed to clarify the rotational vibration behaviour of power transmission helical gear pairs with comparatively narrow facewidth. It has been based on the theoretical deflection solved by one of the authors using the finite difference method. and the rotational vibration has been treated as a single degree of freedom system and the meshing resonance frequency of it has been obtained. Furthermore, its propriety is verified by measuring the acceleration for each gear pair belonging to the three categories classified by contact ratio. it is found that the meshing resonance frequencies calculated by use of the proposed equation agrees with experimental values.