The aim of this randomized clinical trial was to evaluate the retention of single implant mandibular over-dentures retained by locator and ball attachments. 14 completely edentulous patients were included in the study. They received single midline implants to support a mandibular over-denture. Implants were allowed to osseo-integrate for 12 weeks. Patients were randomized into 2 equal groups; six patients in each one. Group I received a ball attachment while group II received locator. Retention was measured using a force meter at day of pick up, 1 month, 3 months and 6 months. Group II Locator showed higher initial retentive force with a mean of (380.25 gm. ± 258.19) than Group I ball with an initial retention of (229.71 gm. ± 73.42). The retention of both attachments decreased with time was no statistically significant difference in percentage of change in retention between them. However, the locator remained more retentive at the end of the follow-up period of 6 months. As a conclusion, the locator showed higher retentive values for the single implant over-denture hen compared to the ball attachment.