Objective: The purpose of this study was to evaluate the fracture resistance and failure modes of onlay restorations prepared with different preparation designs. Materials and Methods: A total of 42 extracted, mandibular first molars (36, 46) were used and divided into six groups according to preparation design, as follows 1A: Anatomic preparation of cusps/ rounded shoulder margin/ occlusal groove; 1B: Flat preparation of cusps/ rounded shoulder margin/ occlusal groove; 2A: Anatomical preparation of cusps/ occlusal groove; 2B: Flat preparation of cusps/ occlusal groove; 3A: Complete anatomical reduction of cusps/ rounded shoulder margin; 3B: Complete flat reduction of cusps/ rounded shoulder margin groups; intact tooth: No preparation. Onlays were constructed with 0.5-mm copings of Zirconia ceramic. The copings were veneered with porcelain (IPS e. max Ceram). All samples were subjected to fracture resistance testing. Data were analyzed with Kruskal-Wallis and Bonferroni-Dunn tests. Results: Fracture resistance varied significantly according to preparation design. Among the anatomic occlusal preparation designs, fracture resistance was significantly lower in Group 3 when compared to Groups 1 and 2 (P < 0.05). Among the flat occlusal preparation designs, fracture resistance was significantly higher in Group 1 when compared to Groups 2 and 3 (P < 0.05). Conclusion: Preparation design affected the fracture resistance of onlay restorations. Cavities with flat occlusal preparation designs, a groove and shoulder margins (1B) resulted in the highest fracture resistance, whereas teeth prepared with a complete reduction of cusps and shoulder margins (3A) had the lowest fracture resistance.