A study in an aggressive environment of the properties of short glass fibre reinforced thermoplastic composites based on poly (ethylene terephthalate) is reported. For a good understanding of hygrothermal ageing mechanisms of these composites an original multi scale approach has been developed. At the molecular scale chemical and morphological changes have been determined through classical tests(，nolecular weight, end chain titration, water uptake, crystalline evolution ...). At the macroscopic scale mechanical properties variations were analysed. In order to link these two scales the local damage were determined through photomechanical tests