The purpose of this study was to examine force transmissions of narrow-diameter ITI implants when used as a terminal support for freestanding three-unit fixed partial dentures (FPD). Photoelastic and strain-gauged models of two 4.1-mm-diameter solid-screw implants; a 3.3-mm reduced-diameter implant and a 4.1-mm-diameter solid-screw implant; and a 3.3-mm-diameter narrow-neck implant and a 4.1-mm-diameter solid-screw implant supporting fixed prostheses were fabricated. A static force of 100 N was applied on both implants and on the pontic of the prostheses in separate load cases, and the generation of isochromatic fringes was observed and photographed in the field of a circular polariscope. The straingauge signals were digitized by a data-acquisition system and displayed in a computer by corresponding software at a sample rate of 10,000 Hz, and the principal strains were calculated. The use of narrow-neck and reduced-diameter implants resulted in an overall increase in stress and strain magnitudes around supporting implants in comparison with support from two standard solid-screw implants. Narrow-diameter ITI implants may be used to support FPDs for patients with low bite forces. In other clinical situations involving narrow-diameter implants, increasing the implant support is crucial to improve the biomechanical outcome of the treatment.