The strengthening mechanism of the glass-fiber at cryogenic temperature has not been fully studied so far. In the present study, tensile tests of a single E-glass fiber with heat treatment in air and liquid nitrogen were conducted to reveal the strengthening mechanism. The strength of glass fiber in liquid nitrogen was twice as high as the strength in air. Based on the area of the mirror zone in the fracture surface, the mirror constant was detemined. Besides, the mirror constant of the glass fiber were equal regardless of the heat treatment condition and the testing temperature. From the observation of surface crack, it was clarified that the mirror zone was the mark of surface flaw propagation and therefore it was suggested that the area of mirror zone doesn't have a direct effect on fiber strength.