The subject of this paper is the Mosco convergence of quasi-regular Dirichlet forms. The author gives a sufficient condition in order that the Mosco limit of a sequence of symmetric quasi-regular Dirichlet forms be quasi-regular. The key point is the uniform tightness of the capacities associated with the corresponding Dirichlet forms. By applying this result, a necessary and sufficient condition is obtained for a Dirichlet form to be a quasi-regular Dirichlet form in terms of its Yosida approximation sequence. The author further studies the question: when is the Mosco limit of a sequence of local quasi-regular Dirichlet forms also a quasi-regular Dirichlet form.