Purpose: to evaluate the effect of using two treatment modalities on the alveolar bone height in flat ridge cases. Materials and methods: Ten completely edentulous male patients were selected. Age of the patients ranged from 55-65 years old. All patients had resorbed lower ridges. The patients were divided into two groups, five patients each. All group (A) Patients received conventional flexible denture while, all group (B) patients received overdenture supported by single sympheseal implant. Mandibular bone height was measured by Cone Beam Computerized Tomography (CBCT) for both groups using stent containing radio-opaque stainless steel wires at posterior region corresponding to (5, 6,7) denture teeth at the time of flexible denture/overdenture delivery and 6, 9, 12 month after. Results: Regarding intergroup comparison of the change in bone height (%) group (A) had a higher mean value of bone height changes than group (B) which was found statistically not significant. Regarding frequency and percentage values for patient satisfaction questionnaire answers there were no significant differences in the answers of both groups. Conclusion: Within the limitation of this study, it could be concluded that, there was no significant difference between flexible complete denture group and single implant flexible overdenture group regarding posterior mandibular bone resorption and patient satisfaction.