Introduction: Evaluation of effect of different posts arid composite restoration on strength of endocontically premolar teeth. Materials and Methods: 90 intact Premolars were selected thirty, as the control group was the experimental group. Control group was divided to three group of ten.One group of ten as Positive (intact teeth) and two groups < of ten as negative (Prepared teeth for class I and II).After RCT, extended class I and II cavity preparation with 3 millimeter Buccoligunal distance were divided to two groups on the basis of class I and II composite restoration.Then each group was divided into three subgroups, On the basis of the type of post:(parapost = P, Bioglass Post= B and composite Post=C).After treating the coronal and radicular Dentin with Scotch Bond Multi Purpose (3 MCo), Parapets was put in to the 2-3 of canal length with a self cured composite (Medental, McAllen), while Biogalss and Composite posts were put in vivadent and self cured composite (Medental, McAllen) After 500 thermal cycling, Breaking test was done with a semi spherical crosshead speed of 5 mm/min.Analysis of data was done by ANOVA, Duncan test and Mann- whiney test with 95% significance level.Results: 1-Mean strength for class I and II restorations were respectively 101.8 and 89.9Kg 2- Mean strength for posts (B, Cand P) were respectively: 95, 48-101.24 and 90.88.Kg 3-There was not a significant difference in strength of restored teeth between class I and II composite restoration4- Strength of teeth significantly increased by post insertion.5-Intact teeth were significantly stronger than experimental group.6-The strength of restored teeth was 50% of intact teethes strength:7-Class I restored teeth were 183% more stronger than teeth with class I cavity prep.8-Class II restored teeth were 56% stronger their teeth with class II cavity prep.Conclusion: If posts are used in endodontically premolar teeth by adhesive technician, and composite is used for restoration, they will have successful clinical performance.