Purpose: Despite the high fracture strength of modern dental core ceramics, fractures are a common cause of clinical failures. The aim of this study was to use a clinically relevant test method to evaluate the effect of the curvature of the cervical crown margins on load at fracture. Materials and Methods: Thirty zirconia crowns and 30 glass-ceramic crowns were produced for three premolar preparations with variation in the approximal crown margin curvature: low curvature (r = 12 mm), moderate curvature (r = 2.5 mm), and high curvature (r = 1.5). The crowns were loaded until fracture with a method that mimics clinical fracture modes. Results: Statistically significant differences were found among both crown margin curvature and material groups (P .001). Most fractures originated from pre-existing flaws. Conclusion: A moderate to low curvature of the crown margins increases crown strength compared with a high curvature.