Objective: The masticatory function of implant-retained overdentures with different attachments has not been sufficiently investigated. This study evaluated the masticatory function of implant-retained mandibular overdentures with ball and resilient telescopic attachments. Method and Materials: Twelve edentulous participants were rehabilitated with new maxillary and mandibular conventional dentures (CD). Three months later, two implants were installed in the canine regions of the mandible. Following a 3-month healing period, duplicate overdentures were constructed for all patients. In a simple random method, six participants were first given ball-retained overdentures (BOD) and the other six received resilient telescopic-retained overdentures (TOD). After testing these prostheses, the first group received the TOD, while the second group received the BOD. Masticatory functions (chewing efficiency [unmixed fraction, UF] and electromyographic activity [EMG] of masseter muscles) were measured 3 months after wearing each of the following dentures: CD, BOD, and TOD. UF was measured using chewing gum and EMG was recorded for right and left bellies of masseter muscle during clenching with (soft and tough) and without foods. Results: Regardless of attachment type, implant-retained overdentures demonstrated significant decrease in UF and significant increase in EMG compared to CD. TOD recorded significantly lower UF and significantly higher EMG when compared to BOD. Conclusion: The TOD improves masticatory functions when compared to BOD. Such improvement may be related to the increased retention and stability of these dentures. Keywords: chewing efficiency, EMG activity, implant, overdenture, telescopic attachments