Statement of problem: Some clinical situations require occlusal correction with occlusal veneers as a conservative treatment approach. The exact veneer thickness that provides both preservation of tooth structure and high fracture resistance for glass ceramics and resin nanoceramics is still under investigation.
Purpose: The objective of the study was to evaluate fracture resistance of occlusal veneers fabricated from two ceramic materials (IPS e.max CAD and Lava Ultimate) and two thicknesses (0.5 and 1mm) after thermocycling.

Materials and Methods:
Forty caries-free maxillary human premolars were prepared to receive occlusal veneers. The veneers were milled from two different materials: IPS e.max CAD (group E, n =20) and Lava Ultimate (Group L, n =20). Half of each group was milled with 0.5 mm thickness and the other half with 1 mm thickness. Each veneer was cemented using self-adhesive resin cement. All samples were subjected to thermocycling for 5000 cycles between 5±2°c and 55±2°c with a dwell time of 30 seconds in each bath and 20-second interval baths at ambient air. Then, all samples were individually mounted on a computer-controlled universal testing machine for fracture resistance testing.
Results:
IPS. emax CAD veneers with 0.5 mm thickness had a significantly higher value than Lava Ultimate . Lava Ultimate 1 mm thickness occlusal veneers showed a significantly higher value than 0.5 mm ones.
Conclusions: All tested occlusal veneers exhibited fracture resistance values within average normal biting forces in maxillary premolar region. IPS.emax CAD ceramic is the material of choice for high strength conservative occlusal veneers.