Unidirectional (UD) composite laminates based on glass fibers (GF) and high-performance polythylene fibers (PEF) were prepared with partially polymerized methyl methacrylate (MMA) at room temperature, followed by heating at 55 degrees C (well below the softening point of PEF) for 2 h. The tensile strength, modulus of elasticity, fiber efficiency and strength efficiency of both the composite laminates, loaded parallel to the fibers, at the same volume fraction range, were investigated. All the properties were compared between the two composite laminates. It was observed that the measured tensile strength and modulus of elasticity deviated from the values calculated from the Rule of Mixture (ROM). The deviation was minimal at the lower volume fraction of fibers, and increased with the fiber volume. An interesting feature that was observed was that the efficiencies of PEF-reinforced composite was higher than that of the GF-reinforced composite at the same volume fraction of the fibers. (C) 2000 John Wiley & Sons, Inc.