A method for measurement of bubble diameter in the flat glass is presentecl. With parallel He-Ne laser beam irradiating the bubble, circular interference fringe is produced in the far-field. Based on the far-field theoretical model, the angle position of interference fringes outside the flat glass is measured with spectrometer and converted into the angle position of interference fringes inside the flat glass according to Snell’s law and then the bubble diameter is calculated with mathematica. The measurement results indicate the bubble in the flat glass is ellipsoid. Suggestion of rapid measurement of bubble diameter is given. Because of far-field interference, the demand for the bubble location over platform is not strict. The interference caused by Fraunhofer screen diffraction fringes is analyzed and a completely new method is provided for measurement of the bubble diameter in the flat glass.