Abstract: A fracture mechanics analysis of bridge effect on a fiber reinforced composite containing a penny crack is presented. The integral equation governing bridge-toughening as well as crack opening displacement (COD) for the composite with interfacial layer are derived from the Castigliano's theorem and interface shear-lag model. A numerical result of the COD equation is obtained according to iteration solution of the second Fredholm integral equation. In order to investigate the effect of the various parameters on the toughening of composite. An approximate analytical solution of the equation is provided and its error analysis is performed. A parametric study is therefore conducted of the influence of the crack length, interfacial shear modulus, thickness of interphase,fiber radius,fiber volume fraction and properties of material on composite toughening.