Background: Occlusal veneers fabricated with different materials and thicknesses can reveal different marginal adaptation after thermodynamic aging. Methodology: Extracted maxillary premolar teeth (n=28) were collected and embedded in epoxy resin block with light body material then randomized according to materials into two groups (n=14):  Lithium disilicate IPS e.max CAD (A) and Zirconia reinforced lithium silicate  Celtra Duo (B) then further randomized into four subgroups (n=7) according to occlusal veneer thicknesses 1 mm and 1.5 mm. Teeth were prepared by a customized milling machine then the exposed dentine was immediately sealed by dentine bonding agent. Fabrication of the occlusal veneers was done by CAD/CAM milling machine. Occlusal veneers were adhesively cemented by dual cure resin cement (Panavia F 2.0). All samples were subjected to thermocycling and cyclic loading equivalent to one year of service (150,000). Marginal gap measurements then were done with A digital microscope at 35x magnification was used to photograph each specimen. Marginal gap was measured and evaluated using a computerized image analysis system. Results: two-way ANOVA showed that the materials only IPS e.max CAD (39.30±5.14) had significantly higher value than Celtra Duo (31.52 ± 5.69) (p<0.001). While regarding the thicknesses 1 mm thick samples (36.78 ± 6.06) had significantly higher value than 1.5 mm thick samples (34.04 ± 7.02) (p=0.020). Conclusion: Zirconia reinforced lithium silicate (Celtra DUO) occlusal veneers showed better marginal adaptation than lithium disilicate (IPS e.max CAD) in different thicknesses. Zirconia reinforced lithium silicate (Celtra DUO) marginal adaptation was influenced by occlusal veneer thickness (the thicker 1.5 mm showed better adaptation than 1mm.