Today the removable partial dentures are used as permanent prosthetic devices. Thus, their designs must be considered the prevention of breakage or deformation. A metal plate denture consisting of double structure has been designed with the intention of improved mechanical strength and durability. Principal investigations of this study were to introduce structural design techniques and apply them to denture design, to analyze the double structure using the finite element method, to judge reliability using a statical bending test, to compare the breaking strength of each structure, and to measure movement during function of mandibular extension base removable partial dentures on the simulation model. The summary of results is shown below: 1. Calculated values and experimental values were a fairly good approximation. 2. The reinforcement and Lightweight method of double structure was made evident. 3. Regarding the packing resins, METADENT was slightly superior in mechanical strength to ACRON. 4. Maximum stiffness of the double structure was approximately 7-10 times that of skeleton types, and breaking strength was about 9 times. 5. Tensile strain generated at the double structure was statistical less than for skeleton types. 6. When loads of 10 kg applied, the difference of distal displacement between double structure and skeleton types was between 70-80 microns. 7. The stiffness of the skeleton types decreased remarkably by shortening the denture flanges, but the double structure did not show significant change.