A mounting structure is provided between internal and external crowns of a prosthetic device, in which a tenon-mortise type engagement is formed for fixing the crowns. The internal crown forms an internal coupling section and the external crown forms an external coupling section that corresponds to the internal coupling section. The external coupling section includes a liftable structure, whereby when the external crown is fit to the internal crown, the liftable structure is slightly bulged outward to allow for easy movement of the external crown for fitting to the internal crown. With the inter-engagement between the internal and external coupling sections, the external crown is set stably and securely on the internal crown and problems of insufficient or excessive fitting tightness between the crowns are eliminated or alleviated.
