Abstract: The strength behaviours of the window glass of spacecraft were discussed based on the strength theories in this paper. It is pointed out that the strength design for the window glass must be based on the fracture mechanics, not just the data of the tensile strength or bending strength measured on small samples Because the cracks existing on glass surface will greatly influence the strength of the window glass, they must be controlled and inspected carefully, so as to ensure the strength of window glass to meet the requirements of application. Considering the subcritical growth of the cracks in glass, the window glass should not be loaded in atmosphere before its servies. In the structural design of the windows, it should be noted that the surfaces in high tensile stress should not contact with moisture.