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Introduction: Severely atrophied mandibles are a common problem within elderly completely edentulous patients. Given their small diameters, long height and self-tapping characteristics, miniimplant retained over-dentures are a valuable treatment option in such patients. Mini-implants are usually once-piece implants, however recently two-piece mini-implants were introduced with aim of addressing the angulation problems. The success of the latter has not been thoroughly studied. Crestal bone height measurements are normally used to evaluate implant success. Therefore this study was conducted to evaluate the effect of one piece versus two piece ERA mini implants on crestal bone height of implant retained mandibular over-denture. Materials and methods: Twelve male, completely edentulous patients ranging between 60-70 years were selected and randomly divided into 2 groups. Group A: all patients were rehabilitated with 4 one-piece ERA mini-implant retained mandibular over-dentures and upper complete dentures. Group B: all patients were rehabilitated with 4 two-piece ERA mini-implant retained mandibular over-dentures and upper complete dentures. Immediate loading was performed and crestal bone height measurements were conducted using Cone Beam Computed Tomography (CBCT) at time of loading, 6 and 12 months following denture insertion. Results: No signs of implant failure (inflammation, infection, radiolucency and mobility) were observed in both groups. Both groups showed statistically significant decrease in peri-implant bone height that was with-in the normal acceptable range (less than 1.5 mm within fist year). There was no statistically significant difference between amounts of bone loss between both groups during the follow up period. Conclusion: According to the results obtained from the study it could be concluded that:  Both one and two piece ERA mini implants fulfil the criteria of implant success as indicated by clinical examination and the measured amount of bone loss.  Similar patterns of bone loss occur when using one piece or two piece (angle correction) ERA mini implants in retaining mandibular overdentures.