Aim; The objective of this study was to evaluate the retentive force of clasps made from two thermoplastic resins by the insertion/removal test simulating two years. Methods: Over standardized premolar and molar metal crowns 36 clasps were fabricated, including 18 clasps (1.5 mm thick) from each of the following thermoplastic resins: polyetheretherketon (PEEK) and polyoxymethylene (POM). Each clasp assembly was subjected to an insertion/removal test on its abutment crown. Retention of each clasp at pre-test (Baseline) and after insertion/removal cycling intervals (360, 730, 1080, 1440, 2116 and 2880 cycles by chewing simulator) were measured by applying withdrawal force to it using universal testing machine. To analyze the retention over the course of insertion/removal test, retention was measured every 360 cycles. Data were statistically analyzed using 3-way ANOVA (α= 0.05). Results. PEEK Resin material group recorded statistically significant higher mean value than Acetal material group mean value (P=0.0013<0.05). Molar abutment group recorded statistically non-significant higher mean value than premolar abutment group mean value (P= 0.0620>0.05). Retention decreased significantly as indicated by three-way ANOVA (P= 0.007<0.05). Conclusion: Retention forces of PEEK clasps were clinically acceptable, and superior to retention forces of Acetal clasps. However, both the retention of adequately designed resin clasps might be enough for clinical use. Clasps made of PEEK showed the more promising retentive force with a non-fracture tendency at tooth undercut.