Abstract: The aging behavior of polyurethane coating through thermal oxidation,ultraviolet exposure and soaking in the water was studied.The mechanical properties were tested and the morphologies of these coatings were determined by means of Scanning Electron Microscopy.The results show that mechanical properties and surface toughness maintained well in 2 weeks in water.But after 3 weeks in water,mechanical properties of the coatings were reduced a lot and some flaws were found on the surface.The micrographs,which were obtained by Scanning Electron Microscopy,show that a lot of cavities in coatings appeared after aging process,and also when the aging time was prolonged,the cavity rate increased,and it penetrated the whole coating finally.