The Gaussian beam divergence for optical resonators (external divergence angle) is not equal to that in free space (internal divergence angle, as a definition) when the change of beam parameter through a dielectric interface has been considered. In this paper, using ABCD-matrix, g-and g*-parameterg, the formulae for external and internal divergence angles in fundamental mode operation and multimode operation have been derived analytically. Four typical examples have been compared and discussed: (l)optical resonators of solid-state lasers; (2) resonators with one plane mirror; (3) directly coated laser rods; (4) resonators with a thermal lens.