Objective: to evaluate the effect of bar and clip attachment systems with different materials on retention forces for two implant supported mandibular over dentures after wear simulation. Materials and methods: An edentulous mandibular model was constructed incorporating 2 parallel implants placed in the canine regions. Attachments were embedded in ametal-reinforced experimental over denture designed to be dislodged from the model by a universaltesting machine. Tensile dislodging force was applied to the overdenture at a cross-head speed of 50 mm/min. Four over dentures were constructed for the attachment systems. The attachments evaluated were metal bar with clip from polyoxymethylene (POM), metal bar with clip from polyetheretherketon (PEEK), (PEEK) bar with (POM) clip and(PEEK) barwith (PEEK) clip. Peak load-to-dislodgement was measured. Retention strength values (N) were recorded initially and after 270, 540, 810 and 1080 cycles of insertion and removal simulating 0, 3, 6, 9 and 12 months of denture using repictively. Repeated measure ANOVA was used to compare between more than two groups for dependent samples while One-way ANOVA was used to compare between more than two groups for independent samples. The significance level was set at P ≤ 0.05. Statistical analysis was performed with IBM® SPSS® Statistics Version 20 for Windows. Results: A statistically significant difference was found between (Metal bar and POM clip), (Metal bar and PEEK clip), (PEEK bar and POM clip) and (PEEK bar and PEEK clip) where (p≤0.001). The highest mean value of retention was found in (PEEK bar and PEEK clip) followed by (Metal bar and PEEK clip) and (PEEK bar and POM clip) while the least mean value of retention was found in (Metal bar and POM clip).Conclusion: Attachments generally demonstrated significantly decreased retention strength values as the cycle's number increased. POM as well as PEEK materials may fulfill the requirements of retentive clips on metal or PEEK bars. The invitro performance of retentive clips made of PEEK is superior to those made of POM.