The purpose of this study was to study the effect of crown thickness on the fatigue life of CAD/CAM ceramic materials. CAD/CAM ceramic materials for the crown were virtually designed with three thickness designs of (a) ultra-thin occlusal crown average 0.7 mm thick (group A), (b) thin occlusal crown 1.1 mm average thick (group B), (c) thick occlusal crown 1.5 mm thick. The materials are: zirconia Cercon ZC and IPS e.max CAD (LD). Finite Element Analysis (FEA) simulations were carried out to estimate the fatigue lives of restorative materials. The lives for groups B and C under fatigue load were not significantly different from each other for Zirconia. The predicted lives for group A zirconia crowns, under fatigue load 50 N, 100 N, 120 N is 24 years, 4.3 years, 1.9 years, respectively. Results for crowns made of LD can be summarized as follows: under load 50 N, all groups have survived longer than 5 respectively, while under the load of 100 N, only group C survived longer than 5 years. 0.7 mm thick full contour Zirconia crowns possessed adequate endurance strength to survive under physiologic conditions. On the other hand, the crown made of LD should have at least 1.5 mm thickness to survive longer than 5 years.