A series of experiments have been performed to study the breakage behavior of float glass. Time and temperature difference at the 1 breakage are the most important parameters to character the glazing behaviors. The temperature differences between the center of fire-exposing side and ambient side are calculated, the temperature differences between the center of fire-exposing side and the shaded region of ambient side are obtained too. The time at 1st crack is also recorded by the camera. It is indicated that the locations of the first crack by the radiation from heat radiating sources are different with the locations heated by the fire which generates the black smoke in the upper of the compartment. The 1 breakage time at thickness of 19 mm could be longer than the other four thicknesses, while the time of glass at the 1 breakage with the thicknesses of 6 mm, 10 mm and 12 mm are basically the same. The temperature differences could increase as increasing the thickness of glass. The results of the critical time and temperature difference are given as a reference value to predict the first fracture in engineering application.