Isothermal crystallization of iPP in model glass-fiber composites is studied by DSC, and the basic energetic parameters of crystallization are determined. Unsized untreated and thermally treated glass fibers are used in model composites to determine the role of the surface on nucleation and crystallization processes. Thermally treated glass fibers are found to exhibit a predominant nucleating effect as compared to unsized untreated ones, and the crystallization proceeds faster, resulting in lower values for the halftime of crystallization (10-120 s). The energy of formation of a nuclei of critical dimensions at a given T-c is also lower, and it decreases as the content of the fibers in the composite increases. The surface free energy of folding, sigma(e) = 140 x 10(-3) J/m(2), was determined for iPP in the composite containing 50% glass fibers, while for pure iPP, sigma(e) = 170 x 10(-3)J/m(2) was found.