Objectives: The aim of this study was to evaluate the effect of two different attachment systems on the retention and implant stability of implant retained mandibular Overdenture. Material and Methods: Fourteen completely edentulous patients with age ranged from 47-65 years planned to receive conventional complete denture. Following the treatment protocol, each patient received two implants in the mandibular anterior region and after insurance of the osseointegration, the patients were randomly divided into two groups; Group (A) received Ball/O-Ring attachment and Group (B) received Locator attachment. The retention of two groups were assessed by the digital forcemeter at 3 times (T); (T0) retention of the conventional complete denture, (T1) at time of insertion of implant retained mandibular overdenture and (T3) retention after three months of insertion of implant retained mandibular overdenture. The implant stability quotient (ISQ) was done using Magnetic Resonance Frequency Analyzer (Osstell, ISQ) at the time of loading then after 3 and 6 months. Results: The retention values before the insertion of overdenture (T0) were considerably low in comparison with those at time of insertion of overdenture (T1) and after 3 months from denture insertion (T3). Regarding the ISQ values, there were no significant difference between the two groups before and at the time of insertion of the implant while there was a significant difference between two groups with better stability resulted in the Locator attachment group after three months (p