Abstract: The natural blue, colourless topazs and irradiated blue topaz are tested by using UV lamp (λ=365, 254 nm). The results show that the UV fluorescences of the natural blue topazs are strong to very weak, and those of the irradiated blue topazs are very weak or even no fluorescences. On the basis of UV fluorescence test, their cathodoluminescence characters are studied emphasisly by using By-1 cathodoluminescence spectroscope produced by the author and USB2000 CCD multi-channel spectroscope produced by Ocean Optics, U.S. The results indicate that the cathodoluminescence of the natural blue topaz is the strongest, that of the natural colourless topaz takes the second place and that of the irradiated blue topaz is the weakest, only 1/3 of that of the natural blue topaz. The cathodoluminescence characters can be used to separate the natural blue topaz from the irradiated blue topaz rapidly and nondestructively.