All-polypropylene (PP/PP) composites, consisting of homoisotactic PP fibers and a propylene-ethylene random copolymer matrix, were manufactured. The interfacial morphologies and mechanical deformation of the composites were investigated. It was found that a transcrystalline layer was formed in the matrix depending on the molding conditions. This layer is composed of two regions with different morphologies: In one region close (<300 nm) to the fibers, lamellae exist very densely and the c-axes of the crystal lattices are oriented along the longitudinal direction of the fiber. In the other region, a little away (>300 nm) from the fiber, a cross-hatched structure was observed. The well-developed transcrystalline layer could be highly elongated, and no debonding was observed at the fiber/matrix interface by the tensile stress in the direction perpendicular to the fiber. It is conceivable that this is the reason for the observed improvement of the transverse tensile properties in the PP/PP composite with a transcrystalline layer. (C) 2003 Wiley Periodicals, Inc.