Mohamed Abdelhaleem Nagib¹, Mostafa A Elsayad², Ashraf Gibreel³ and Tamer M Belal ^{4 1}B.D.S Faculty of dentistry Sinai University, clinical demonstrator of prosthodontics faculty of oral and dental medicine Delta university ²Associate of prosthodontics faculty of dentistry Mansoura university ³ Assistant professor of prosthodontics faculty of dentistry ,Mansoura university ⁴ Assistant professor of Neurology Faculty of Medicine ,Mansoura university Abstract: to evaluate electromyographic activity of masseter muscle with two different prosthesis (fixed denture and milled bar over-denture) constructed over four implants placed according to all on four concept. Materials and methods: Ten completely edentulous patients) age ranged between 55-65 years). All selected patients received new maxillary and mandibular dentures (CD) After 3 months, all patients received 4 implants according to the All on four concept  and the implants were immediately loaded with lower denture  ,After 3 months of healing, Cross over (within patient) study design was started,each patient received the following prostheses in the mandibular arch: Fixed complete hybrid denture (FD) and Milled bar over-denture (MB), every 3 monthes we measure electromyograph for each prosthesis Results: Muscle activity and chewing ability increased milled bar overdenture and hybrid denture. Also chewing time was significantly decreased. The highest muscle activity was observed in the group of patients treated with group milled bar overdenture. Conclusion: Both Fixed hybrid dentures and milled bar overdentures significantly improve masseter muscle activity compared to conventional dentures when used as a definitive prosthesis for patients rehabilitated according to “All on four concept"Milled bar overdentures were associated with more muscle activity, chewing rate during chewing hard food than fixed hybrid dentures   Keywords: electromyography, all on four concept, milled bar, hybrid denture.