As the first step of Self-luminous Glass Tube (SLGT) mass production and DB construction, the characterization and the optimization of a phosphor coating were attempted by using a cathodoluminescence (CL) device. The experiment was divided into three parts: measurement of the relative luminance at various exciting conditions, measurement of the absolute luminance at 4 keV, and a degradation experiment. The relative luminances were measured at various conditions with different phosphor thicknesses, energies and current densities. Regardless of the energies, as the current density increased, and the thickness decreased, the relative luminance increased. The absolute luminance was measured at only 4 keV. Absolute luminance at other energies was estimated from relative luminance data by using a conversion factor calculated from absolute and relative luminance measured at 4 keV. We tried to estimate the expected life of SLGT, which is limited by the reduction of the phosphor efficiency and the amounts of tritium, by additional degradation experiments under severe conditions. The luminance of a 10 μm film was much higher than that of a 20 and 45 μm at the initial stages. On the other hand, the decreasing rate of the luminance at 10 μm was more drastic than that of a 20 and 45 μm. We could see burnt-out spots only on the 10 μm samples. The other samples (20 and 45 μm) were not showing any burnt-out spots.