Objective: To objectively evaluate the effect of mini-implant-supported mandibular overdentures on electromyographic activity (EMG) of the masseter muscle during chewing of hard and soft foods. Method and Materials: Twelve completely edentulous patients (4 females and 8 males) with maladaptive experience of wearing mandibular dentures received new maxillary and mandibular dentures. After 3 months of adaptation, four mini dental implants (MDIs) were inserted in the interforaminal region of the mandible, and the new mandibular dentures were connected to the implants immediately with O/ring attachments. The activity of masseter muscle (EMG) and the duration of chewing cycle were measured during chewing hard (carrot) and soft (gum) foods. The measurements were made 3 months after wearing each of the following prostheses: the new conventional dentures; and the MDI-retained mandibular overdentures. Results: The EMG of masseter muscle increased and the DC decreased with MDI-retained mandibular overdentures when compared to conventional dentures. Hard food (carrot) was associated with increased EMG and decreased DC when compared to soft food (gum) for both conventional dentures and MDI-retained mandibular overdentures. Conclusion: Mini-implant-supported mandibular overdentures are associated with increased activity of masseter muscle and decreased duration of chewing cycle for both hard and soft foods when compared to conventional dentures. Keywords: chewing, EMG activity, mini-implant, overdenture