Objective To compare the fracture resistances of teeth restored with 3 different posts-core and failure modes. Methods 54lower premolarsendodontically treated teeth were selected for thisstudy and divided into three groups randomly (without ferrule, 1 mm ferrule and 2 mm ferrule). Each group was then divided into 3 sub-groups (6 teeth in each group) and restored with cast post-core, glass fiberpost-core, andceramic post-core respectively. After thermocycling specimens , the fracture resistance was measured using a cyclic fatigue testing machine. Data were analyzed with 2-way ANOVA (α = 0.05). Results Fiber post-core group exhibited significantly higher resistance to fracture compared with groups of cast post-core and ceramic post-core (P<0.05). Preparing dentin ferrules increased significantly fatigue resistance (P < 0.05). Conclusions Glassfiber posts were able to preserve and reinforce the remaining tooth structures. Dentin ferrule preparation is necessary to enhancefractureresistance.