to study effect of different preparation designs on the marginal adaptation of lithium disilicate endocrowns. Materials and methods: Twenty human mandibular molars were selected to conduct the present study. All teeth were randomly divided into 2 groups (n=10 each) according to the preparation design of endocrown with two coronal cavity depths: Group (2D): Endodontically treated teeth with 2mm depth intra-coronal cavity preparation .Group (4D): Endodontically treated teeth with 4mm depth intra-coronal cavity preparation. Samples of each group were further subdivided into 2 subgroups (n=5 each) according to the preparation's margin configuration: Subgroup (B): preparations with butt-joint margin configuration. Subgroup (S): preparations with 90o shoulder margin configuration. All prepared teeth were restored using IPS e.max Press endocrowns. The vertical marginal adaptation was measured after cementation using a Stereomicroscope. Measurements were made at twenty points for each crown. The recorded data were collected, tabulated and statistically analyzed. Results: shoulder finish line marginal preparation recorded lower vertical marginal gap values than butt joint marginal preparation. 2mm intracoronal cavity depth extension recorded lower vertical marginal gap values than 4mm intracoronal cavity depth extension. Conclusions: All obtained marginal adaptation values lie within the clinically accepted ranges. Increasing intra-coronal cavity depth of endocrown preparation negatively affects marginal adaptation of lithium disilicate based endocrowns. Endocrown with shoulder finish line marginal configuration has superior marginal adaptation than those with butt joint marginal configuration.