Unidirectional composite laminates based on carbon fibers (CF) and highperformance polyethylene fibers (PEF) and their hybrids 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, 147 degrees C) for 2 h. The stress-relaxation behavior of the composites were determined and analyzed. It was found that at all strain levels the rate of stress relaxation decreased by the loading of CF in CF-reinforced composite laminates (CFRC); however, the reverse behavior was found in the case of PEF-reinforced composite laminate (PEFRC). An interesting observation of the study was that the rate of stress relaxation decreased linearly in two steps in the case of PEFRC, whereas in the case of CFRC, it decreased in a single step. In the case of hybrid composites, the stress relaxation decreases in two steps as in PEFRC.