Objectives:: The current study aimed to assess marginal adaptation of polymer infiltrated ceramic (Vita Enamic) and Zirconia reinforced lithium silicate ceramic (Vita Suprinity) onlays with palatal anatomical occlusal preparations. Materials and Methods: In this study 20 Caries free human maxillary premolars were used with a standard onlay cavity preparation with anatomical occlusal preparation on the palatal cusp. Preparations were scanned, designed and sent for milling the ceramic onlays using CORiTECH 250i milling machine. Half of onlays were milled from Vita Enamic blocks and the other half from Vita Suprinity blocks.  Resin cementation weas done for all the onlays. The onlays were subjected to 5000 cycles of thermocycling. Marginal gaps (µm) were measured using digital microscope before and after thermocycling. Three-way ANOVA was used to study the effect of tested variable and its interaction on marginal gap (µm). Results: Zirconia reinforced lithium silicate ceramic (Vita Suprinity) had higher marginal gap than Polymer infiltrated ceramic (Vita Enamic) before and after thermocycling (40.52µm) (30.29µm) and (71.86µm) (62.10µm)    respectively. Also, thermocycling had increased the marginal gaps for all onlays sample. Conclusion: Polymer infiltrated ceramic (Vita Enamic) onlays had better marginal adaptation compared to Zirconia reinforced lithium silicate ceramic (Vita Suprinity) onlays before and after thermocycling using anatomical occlusal preparations.