Unidirectional composite laminates based on glass fibres (GF) and high performance polyethylene fibres (PEF) and their hybrids were prepared with partially polymerized methyl methacrylate at room temperature, followed by heating at 55 degrees C (well below the softening point of PEF) for 2 h. The stress relaxation behaviour of the composites were determined and analysed. It was found that at all strain levels, the rate of stress relaxation decreased by incorporation of GF in GF-reinforced composite laminates (GFRC) but the reversed behaviour 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 GFRC, it decreased in one step. The rate of stress relaxation was increased with the increase of relative proportion of PEF in the hybrid composites and at a higher proportion of PEF, it decreased in two steps like PEFRC.