The aim of this study is to evaluate the effect of the humidity on the long term behaviour of glass fiber reinforced thermoplastic in fatigue. Two sets of samples were studied, one set contained 0.2 wt% of water, the second 3.5 wt%. The fatigue tests are performed at a 10 Hz frequency, at room temperature and two various relative humidity ratios, 50% RH and 96% RH. The S-N curve of dried samples (0.2%) is above the one of humid samples (3.5%), the endurance limit at 10(7)supercript stop cycles for dried samples is equal to 40 MPa against 35 MPa for the second set. For a given strain, the fatigue life is higher for humid samples because the induced stress is much lower due to the plasticizing effect of water. Though the tests are carried out at room temperature (23 degrees C), the sample temperature at the surface reaches values higher than Tg and whatever the applied strain, the matrix is in a rubbery state when the fracture occurs. On the basis of S.E.M. examinations, the following scenario is proposed: crack initiation at the fiber end, crack propagation along the fiber sides going with debonding, then crack propagation in the rubbery matrix.