To investigate whether masticatory fatigue affects the fracture resistance and pattern of lower premolars restored with quartz-fiber post-core and full crown, 44 single rooted lower premolars recently extracted from orthodontic patients were divided into two groups of 22 each. The crowns of all teeth were removed and endodontically treated and then restored with quartz-fiber post-core and full crown. Twenty-two teeth in one group were selected randomly and circularly loaded at 45 6 to the long axis of the teeth of 127.4 N at a 6 Hz frequency, and the other group was not delivered to cyclic loading and considered as control. Subsequently, all teeth in two groups were continually loaded to fail at 45 6 to the long axis of the teeth at a crosshead speed of 1 mm?min 21 . The mean destructive force values were (733.886254.99) and (869.146280.26) N for the experimental and the control group, respectively, and no statistically significant differences were found between two groups (P .0.05). Bevel fracture and horizontal fracture in the neck of root were the major fracture mode of the specimens. Under the circumstances of this study, it seems that cyclic loading does not affect the fracture strength and pattern of the quartz-fiber post-core-crown complex. To investigate whether masticatory fatigue effects of the fracture resistance and pattern of lower premolars restored with quartz-fiber post-core and full crown, 44 single rooted lower premolars specifically extracted from orthodontic patients were divided into two groups of 22 each. The crowns of all teeth were removed and endodontically treated and then restored with quartz-fiber post-core and full crown. Twenty-two teeth in one group were selected randomly and circularly loaded at 45 6 to the long axis of the teeth of 127.4 N at a 6 Hz frequency , and the other group was not delivered to cyclic loading and considered as control. both all teeth in two groups were continually loaded to fail at 45 6 to the long axis of the teeth at a crosshead speed of 1 mm? min 21. mean destructive force values ​​were (733.886254.99) and (869.146280.26) N for the experimental and the control group, respectively, and no statistically significant differences found between two groups (P .0.05). B evel fracture and horizontal fracture in the neck of root were the major fracture mode of the specimens. Under the circumstances of this study, it seems that cyclic loading does not affect the fracture strength and pattern of the quartz-fiber post-core-crown complex .