In the present work, the influence of interfacial strength on the fatigue behaviour of longitudinal glass fibre reinforced polypropylene has been examined. The interfacial properties of two composite systems with different matrices were investigated by using microscopic fractography and monitoring by the damage mechanisms with a surface replication technique. The observed differences in fatigue micromechanisms (debonding, fibre breakage, matrix failure etc.) are presented and discussed in reference with the macroscopic fatigue lives and residual stiffness.