The purpose of this in-vitro study was to compare the fracture resistance of occlusal veneers made from hybrid ceramic (Cerasmart) with two different thicknesses. Twenty four human first maxillary molars were used in this study. Extracted maxillary first molars were collected from patients with untreatable periodontal problems (grade III mobility); Teeth were chosen to be free from any caries, defects or Cracks. To facilitate standardization of tooth position during occlusal veneers preparation and during the test procedure, teeth were mounted in epoxy resin blocks. The teeth were position in the center of the block, parallel to its long axis and perpendicular to its base. A specially designed centralizing device was constructed to allow the accurate placement of the teeth during fabrication of the epoxy resin block. Teeth were randomly divided into two main groups (12 each). According to the hybrid ceramic material thicknesses table (2) and Figure (12).Group A - (n= 12) (control) Teeth were prepared to receive Cerasmartocclusal Veneers with 1.8 mm thickness at the cusp tip and 1.2 mm at the central groove.Group B – (n= 12) (intervention): Teeth were prepared to receive Cerasmartocclusal veneers with 1.3 mm thickness at the cusp tip and 0.6 mm at the central groove. A standardized tooth preparation was applied to all specimens using CNC milling machine, with anatomical occlusal surface preparation with 1 mm ferrule and 1mm deep chamfer finish line. Then occlusal veneers were fabricated by CAD/CAM milling using The CEREC AC system. Then all occlusal veneers were checked over their corresponding teeth for Seating. The internal surface of the restorations was rinsed with water and air dried, and then sandblasted with 50 µm aluminum oxide particles. The internal surface of all restorations was then silanized using silane coupling agent for 60 s then air dried for 5 sec. The prepared teeth were washed with water and dried with air only taking care not to over dry the tooth surface. A customized loading device was then used to apply a constant load of 1kgparallel to the long axis of each occlusal veneer to prevent rebounding during cementation using Re1yXTMUnicem (dual cure self-adhesive resin cement).All samples were subjected to thermo-cycling for 1000 cycle, and cyclic loading for 25,000 cycles. The samples testing was done by compressive mode using a universal test machine along the long axis of the occlusal veneers using load applicator in the form of metallic rod with rounded end (5 mm diameter) traveling at cross-head speed of 1mm/min. with tin foil sheet in-between to achieve homogenous stress distribution and minimization of the transmission of local force Peaks. After fracture testing, the fracture mode of all samples was examined visually using magnifying lens and photographed using digital camera.The following results were obtained It was found thatthe group ofocclusal veneers with Thickness of 0.6 and 1.3 mm group recorded statistically significant (P<0.01) higher fracture resistance mean value (2300.5±470 N), when compared to occlusal veneers with Thickness of 1.2 and 1.8 mm group (1743.7± 387N) as indicated by Student t-test.Fracture modeof the specimens It was found that all specimens showed favorable fracture mode I (restorable fracture) i.e. Fracture of restoration only. Inspection of the samples after fracture frequently showed cement bonded to the intaglio of the occlusal veneers only and debonded from tooth surface (adhesive failure).