This study's objective was to ascertain how preparation design affected the preservation of occlusal veneers made of PEEK and lithium disilicate. Materials and Methods: A total of 32 mandibular molars mounted in epoxy resin models along their long axes on the dental surveyor. CAD/CAM blocks teeth were divided into two test groups (n=16) to fabricate the occlusal veneers of 1.0 mm, group (1): PEEK, and group (2): IPS e.max CAD. The preparation design for occlusal veneer was then used to divide each group into two equal subgroups (n=8), where subgroup (A) represented the minimally invasive occlusal veneer preparation resembling advanced occlusal erosion. Occlusal veneer preparation with a marginal chamfer was represented by subgroup (B). After cementation, the Specimens were put through 5000 cycles of cyclic fatigue loading, which is equal to 5 years of clinical service. Using a universal testing machine, the retention values for all occlusal veneers were determined. Results: IPS E.max material had higher significant mean retentive values when compared to PEEK material. Also, the planner finishing had a higher significant mean retention value when compared with the chamfer finishing line design for IPS E.max material, however, no statistically significant difference when using PEEK. Conclusion: Compared to PEEK occlusal veneers, lithium disilicate occlusal veneers offer better retention. Furthermore, occlusal veneers with planner finishing lines exhibited offer better retention than chamfer finishing lines.