Purpose: to study the influence of different preparation designs on fracture resistance and retention of CAD/CAM endocrowns. Materials and methods: Forty human mandibular molars were selected to conduct the present study. All teeth were randomly divided into 2 groups (n=20 each) according to the preparation design of endocrown, Group (1): Endodontically treated teeth without ferrule (butt joint). Group (2): Endodontically treated teeth with 1mm ferrule. All prepared teeth were restored using IPS e-max CAD endocrowns. After cementation of endocrowns all specimens were subjected to thermal cycling in automated thermocycling machine in order to mimic the intra-oral condition. Then retention was measured using material testing machine, also fracture resistance was measured using material testing machine. The recorded data were collected, tabulated and statistically analyzed. Results: Endodontically treated molar with 1mm ferrule showed high fracture resistance and retention than endodontically treated molar without ferrule. Conclusions: All obtained fracture resistance and retention values lie within the clinically accepted ranges, endocrowns with 1mm ferrule have fracture resistance and retention higher than endocrowns without ferrule (butt joint).