Objective · To compare the stress distribution at abutments, edentulous ridge, and peri-implant tissue under the vertical load between the RPI-type removable partial denture supported by implants with different diameter and the conventional RPI-type removable partial denture using a three-dimensional photoelastic method. Methods · The mandibular Kennedy I type denture defect models were fabricated according to the standard mandibular model. Four Straumann system soft tissue level implants with different diameters (two 4.1 mm implants and two 3.3 mm implants) were implanted into the bilateral first molar areas. Planting nails and epoxy resin teeth were inversely put into the mandibular silicone rubber molds to fabricate the epoxy resin photoelastic models and the regular neck (RN) healing abutments were screwed into the implants. A simulated alveolar mucosa with thickness of 2mm and a conventional RPI-type removable partial denture were placed on the epoxy resin models with or without implants. Then a force of 1 kg was vertically applied by a bite force loading device which fixed the model and denture in the centric occlusion. After the stress was frozen, model slices at different areas were cut and the stress fringes were observed. The stress was calculated according to the fringe value of per unit thickness. Results · For the conventional RPI-type removable partial denture, the maximum stress of distal abutment was mainly concentrated on the apical regions and the maximum stress of edentulous ridge was concentrated on the first molar area, particular on the lingual side of alveolar bone. For the implant-supported RPI-type removable partial denture, the maximum stress of distal abutment was lower than that of the conventional RPI-type removable partial denture, and the maximum stress of edentulous ridge (without the implants) was similar to that of abutment and supporting tissues and was lower than that of the conventional RPI-type removable partial denture. The stress around the implant was the largest and the peri-apical stress of implant was greater than the neck stress of implant. Meanwhile, the maximum stress around the implant was increased with the decrease of implant diameter. Conclusion · Compared to the conventional RPI-type removable partial denture, removable partial dentures supported by implants are more stable with a more balanced stress distribution in supporting tissues, thus benefit the health of abutments and supporting tissues.