This study aimed to evaluate the combined effect of fiber post in different Root canal tapers. Sixty lower first premolar human extracted teeth had been used in this study, teeth were randomly divided into 3 equal groups (n=20). Group I; where cleaning and shaping was done using Twisted File (TF) #25 taper 0.04. Group II; where cleaning and shaping was done using Twisted File (TF) #25 taper 0.06. Group III; where cleaning and shaping was done using Twisted File (TF) #25 taper 0.08. Each main group was randomly subdivided into 2 equal subgroups (10 each) according to the coronal restoration type. Subgroup A; where coronal cavities were filled with Composite core material Multicore Flow (Ivoclar Vivadent, Schaan, Liechtenstein) (MCF), Subgroup B; where the coronal cavities were restored with fiber reinforced composite posts (FRC) and MCF. The fracture resistance of all groups had been evaluated & analyzed using Two-Way ANOVA. Results showed that 0.08 taper had a significant negative effect on the fracture resistance of teeth with and without post than 0.04 taper while other variants had no any statistically significant effect on the fracture resistance of the tested samples. It has been concluded that, under limitations of this study,  the degree of taper of the root canal preparation only affects mechanical properties of the tooth when it increases more than 0.06 taper, while wether a post is used or not  is of no use in class I conservative access cavity preparation of lower first premolar teeth.