Aim of study: was to evaluate and compare the effect of three different denture base material (heat cured acrylic resin, cast metal base and thermoplastic resin) on electromyographic activity of masseter and temporalis muscles in maxillary single denture wearers. Material and methods: Ten completely maxillary edentulous patients were selected with ages ranged from 55-60 years. All patients had received heat cured acrylic resin single maxillary complete denture (Group І) then replacing the denture base through rebasing procedures by cobalt chromium (Co-Cr) (Group ІІ) then thermoplastic resin material (Group ІІІ). All patients were instructed to use their resin dentures for equal periods of three months when EMG records of muscles of mastication (masseter and temporalis) were obtained at insertion then patient recalled after one month and at the end of three months , period of two week interval was left between each group as a washing period. Results: The results of this study showed that there were a significant differences between them, as the thermoplastic resin (group ІІІ) was the best followed by cobalt chromium denture (group ІІ) then heat cured acrylic resin denture base (group І) .Conclusion: EMG activity affected by maxillary denture base material as the lesser effort of masticatory muscles was done by using thermoplastic resin denture base rather than Co-Cr or acrylic resin denture base.