Aim of study: was to evaluate and compare the effect of occlusal reactive versus thermoplastic partial denture bases on the supporting alveolar structure of mandibular distal extension partial dentures cases. Materials and methods: Ten partially edentulous patients were precisely selected to avoid factors that may adversely affect the results of this study. Removable partial dentures were constructed for all patients following the same procedures and the same design. According to denture base material the selected patients were randomly divided into two equal groups: Group I: five patients received cobalt-chromium partial denture with thermoplastic denture base. Group II: five patients received cobalt-chromium partial denture with occlusal reactive base. The patients were evaluated both clinically and radiographically, clinical assessment of the abutment was carried out through measuring its pocket depth, while radiographic assessment was done by recording bone height change distal to abutment teeth and bone high change of the residual ridge. Results: The results of this study showed that there were a significant differences between flexible and occlusal reactive bases on their effect on supporting structures. Conclusion: The occlusal reactive base exhibited better effect than flexible base regarding bone resorption in abutment as well as residual ridge