Objectives: This study was conducted to compare implant retained mandibular overdenture with two different attachments (ball and socket and locator).Subject and methods:7 edentulous patients were selected according to certain criteria. Each patient received two platform switched implants placed in the mandibular canine-premolar region bilaterally. GROUP I: Locator attachments were used to retain the implant-supported overdentures. Locator abutments were loaded after 2-3 months.All patients received conventional maxillary dentures. After follow up periods we removed the attachment with wash period 1month then we used ball and socket attachment GROUP II. The two groups were evaluated for retention, masticatory efficiency and EMG at 0, 3 and 6 months after loading of each attachment. Results: regarding the effect of time on retention of both groups in ball group there was a significant increase in retention at base line till 3 months and decrease from 3-6 months, while locator group there was increase of retention through all periods. Regarding the effect of time on both groups the masticatory efficiency of both ball and locator group have no significant difference through all periods. Comparing the two groups the masticatory efficiency has no significant difference except on chewing carrots the numbers of strokes till first swallow increase in ball group, and on chewing peanut after 6 months the ball group show higher time till first swallow than locator as well as the time to empty mouth.Regarding the effect of time on both groups the electromyographic activity of both masseter and temporalis muscles has no significant difference through all periods except in locator group decrease the activity of masseter after 3 months. Conclusions: the ball attachment at the beginning has a higher retention quality than locator attachment, by time ball attachment loss some of its retention rather than locator attachment. The electromyographic activities and the masticatory efficiency of temporalis and masseter muscles of both ball attachment and locator attachment were comparable.