This study was conducted to investigate the influence of core design on fracture resistance for all-ceramics crowns. a stainless-steel metal die was constructed then duplicated to produce stone dies for the construction of all-ceramic crowns, the die was designed to include 1.2mm shoulder finish line, and 60 total wall tapering, a total of fifteen posterior upper first premolar full crown restorations were divided into 3 groups (n=5) each: (1) Group I: core thickness 0.7mm (with 1mm collar all around), and veneer thickness 0.5mm, the core fabricated of all-ceramic zirconia-based crown and veneered with a ceramic (VITA VM®9, Vita zhanfabrik); and (2) Group II: core thickness 0.7mm ( all surface except buccal 0.5mm), and veneer thickness 0.5mm (all surface except buccal 0.7mm), with the same material of group I; and (3) Group III: core thickness 0.5mm (with 3mm palatal collar extended to contact area), and veneer thickness 0.7mm, with the same material of group I and II. Then metal die duplicated to epoxy resin material were used in fracture testing. Specimens were subjected to compressive loading using a universal testing machine at a crosshead speed of 1.0mm/min. A stereomicroscope was used to determined the failure modes of specimens.