Purpose: This study was designed to examine fracture resistances of different monolithic crowns made by computer-aided design and computer-aided manufacturing (CAD/CAM) on dental implants with different occlusal thickness. Materials and methods: 20 molar crowns were fabricated of four different monolithic ceramic blocks (n=5): Group I: Vita Enamic (V.enamic), GroupII: IPS E-max CAD(e.max), GroupIII: Celtra Duo(CD), GroupIV: Functional explore (f.explore);using CAD/CAM system with different occlusal thickness (1 and 1.5mm). All crowns were cemented on implant abutments using self adhesive resin cement. Combined thermal cycling and mechanical loading was performed under a chewing simulator, A universal testing machine was used to apply load to crown until fracture. The fractured specimens were examined with a scanning electron microscopy. Results: The highest fracture resistance was recorded with f.explore group followed by CD group then e.max group while the lowest fracture resistance mean value recorded with V. Enamic with statistical significance (P=0.0001< 0.05) for both occlusal thicknesses. The 1.5 mm occlusal thickness recorded statistically non-significant (P=0.2267 > 0.05) higher fracture resistance mean value than 1mm occlusal thickness except for f.explore group.Conclusions: The fracture resistance of monolithic crowns were differently influenced by the ceramic materials and changes in occlusal thickness. Within the limitations of this study, all the tested crowns withstood the physiological masticatory loads both at the recommended and reduced occlusal thickness.