The failure mechanism of delamination fracture in composites is investigated. A model based on the fracture mechanics principles is applied to describe the in situ delamination failure process. The crack tip plastic zone developed in the thin resin layer between fibers is considered as the major energy dissipation mechanism. The model expresses delamination fracture toughness G_Ic (laminate), Mode I critical strain energy release rate, as a function of several resin properties. The delamination behavior of several epoxy/fiberglass systems was characterized using the width-tapered double cantilever beam method. The G_Ic (laminate) results are correlated with the resin properties based on the in situ failure model. Resin modulus E and yield stress σ_y are found to dictate the translation from G_Ic (resin) to G_Ic (laminate. Residual stresses in the laminates can also significantly weaken the delamination resistance. A comprehensive comparison between laminate and resin fracture toughness reported in the literature is analyzed from the viewpoint of the mechanism modelled.