Implant-retained mandibular overdenture presents a reliable and simple solution to denture retention and stability. The retention and stability characteristics are provided mainly by implants through attachments. So, in this study various types of attachment systems have been used for connecting implant-retained mandibular overdentures to underlying implants. Available restorative space is an important factor in stress distribution of implant-supported overdentures. The purpose of this study was to evaluate the effect of different vertical restorative space, different bar height in bar and clip attachment and different collar heights in ball attachment on the stress distribution around implants. In this in-vitro study, the model of 2-implant supported overdenture with four different attachments (bar and clip, ball, locatorand bar-ball) on an acrylic edentulous mandibular arch was used. Four different bar height (0.5, 1, 1.5, 2 mm) in bar and clip attachment and three different collar height (1, 2, 3 mm) in ball attachment with 15mm occlusal plane height and three different occlusal plane height (9,12,15mm) were modeled. Vertical unilateral and bilateral load of 150N was applied to the central occlusal fossa of the first molar and the stress of bone around implant was analysed by finite element analysis. By increasing vertical restorative space, the maximum stress values around implants were decreased in unilateral loading models but slightly changed in bilateral loading cases. By increasing bar height, the maximum stress values around implants were increased in unilateral loading models but slightly decreased in bilateral loading cases. By decreasing of first lever arm (distance of crestal bone to abutment) in ulinted resilient attachments stresses generated in bone are decreased and by increasing second lever arm (distance of occlusal plane to abutment) stresses in bone was decreased. Locator attachments generated more stresses than the other ettachments and ball attachment generated the lowest stress in the same crown height space (CHS). Most edentulous patients asking for implant-retained overdenture complain from retention and stability of their complete denture due to sever residual ridge resorption. Sever residual ridge resorption contributes to more vertical restorative space. Based on the findings of this study, it would be biomechanically favorable for these patients if the bar height is reduced and overdentures acrylic resin thickness is increased to consequently reduce periimplant bone resorption. Keywords: finite element analysis, implant, attachment, overdenture, stress.