Objective: To investigate the effect of computer aided design/compnter aided manufacturing (CAD/CAM) enamic blocs with different thickness on the fracture load. Methods: Fifty test specimens were fabricated with isolated tooth in the study. These specimens were divided randomly into two groups:the experimental group (Vita enamic)and the control group (Vitablocs MarkⅡ). In each group,these specimens were divided randomly into five differential thickness sub groups (1.0 mm,1.5 mm,2.0 mm,2.5 mm and 3.0 mm). Every specimen was cemented on the differential thickness bloc with panavia f resin adhesive system. They were stored in distilled water at 37℃ for 24 hours prior to fracture testing. Then, the fracture load was tested in a universal testing machine. Results: Accompany with the increase of thickness of bloc,the increase of fracture load was detected in both groups. When the bloc thickness lied from 1 mm to 2 mm,the fracture load in both groups was similar. But from 2.5 to 3 mm,the fracture load of Vita Enamic was higher than that of Vitablocs MarkⅡ . Conclusion: The fracture load between the Vita enamic and Vitablocs MarkⅡ was similar. Only when the thickness is over 2.5 mm,the mechanical property of Vita enamic was super to that of Vitablocs MarkⅡ.