As the development of zirconia crown using CAD/CAM technology, the usage of full zirconia crown is gradually increased. Compare to the layering technique which is fabricated with zirconia coping and veneering porcelain, full zirconia crown shows higher strength and easier laboratory procedure. To prevent mechanical fracture of the full zirconia crown, the thickness of zirconia crown and proper sintering process should be considered. The purpose of this study was to investigate the influence of zirconia thickness for full zirconia crowns on fracture resistance. Mandibular 1st molar resin teeth were prepared the various thickness; (i) group A: 0.5mm, (ii) group B: 1.0mm, (iii) group C: 1.5mm, (iv) group D: 2.0mm, (v) group E: 2.5mm. Different preparation shapes of resin die and full zirconia crowns with different occlusal thickness were fabricated using CAD/CAM system (CAD/CAM system 5-TEC, Zirkonzahn, Italy). Pre-sintered zirconia crowns were sintered for 12 hrs at 1600โ and then glazed. The samples were cemented on the resin die using resin cement. All specimens were stored in distilled water at 37โ for at least 24 hrs and then were tested with a universal testing machine. Single load-to-fracture was applied on the lingual aspect of the disto-facial cusp at a rate of 1mm/min. The specimens were thoroughly evaluated for cracks and/or bulk fracture with SEM and fractography.