This study evaluated the effect of implant diameter on strain around implants retaining mandibular overdentures with Locator attachments. Three mandibular acrylic resin models were constructed with 2 implants inserted in canine areas and classified according to implant diameter into 3 groups: large (4.2 mm), medium (3.7 mm), and small (3.3 mm) diameter implants. Duplicate dentures were connected to the implants with Locator attachments. Four strain gauges were bonded to the acrylic resin at mesial, distal, buccal and lingual surfaces of each implant and strain was measured at loading and non-loading sides during 1st premolar and 1st molar loading. Small and large diameter implants recorded the highest and the lowest strain, respectively. Buccal and lingual sites recorded the highest strain, and mesial site recorded the lowest. First premolar loading recorded significant higher strain than first molar loading. The largest possible implant diameter is recommended to minimize strains around implants retaining mandibular overdentures with Locator attachments.