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The role of interfacial parameters on the fracture toughness of cement based composites are studied by means of a two-crack system. The first crack type represents the interfacial debonding of a fiber using a pullout model, while the second type simulates the crack growth in the matrix response subjected to the closing pressure generated by the fiber pullout force. A fracture mechanics approach is applied to the pullout of the short fiber. The interfacial zone is characterized as an elastic-perfectly plastic one-dimensional layer with a toughness lower than that of matrix or fiber. Stable debonding of fibers was modeled using R -curves so that the Interface toughness increases with an increase in debonded length. The closed-form solutions for strain energy release rate of a partially debonded frictional interface are solved for the critical debonding length. By obtaining the R -curve parameters, the fiber pullout load-slip response is simulated. The R -curve formulation is further applied to the crack growth in the composite. The toughening component is due to the closing pressure of fibers that depends on the matrix crack opening. The stress crack-width relationship is obtained as the crack growth in the stable region takes place. A parametric study of the effects of interfacial parameters on the crack growth in the composite is conducted. The present model is also compared with experimental data on the size effect in geometrically similar specimens.