Abstract: Most gemmological descriptions on the colour of diamond fluorescence is based on observation by naked eye, and lacks of precise quantitative fluorescence colour determination.In this paper, the authors studied the ultraviolet fluorescence colour of natural near-colourless diamond samples by using USB4000 and light-emitting diode (LED) with 365 nm emission.The fluorescence spectra of 45 natural near-colourless diamonds were measured and analyzed by using theory of colorimetry in order to get the chromaticity coordinates and other related colorimetric parameters.From the calculation, the authors divided the fluorescent colour of diamonds into three categories.Under the same intensity of 365 nm ultraviolet excitation, blue fluorescence in both saturation and intensity are highest among other fluorescence colours.Under microscopic observation, fluorescence colour of diamonds showed blue, pink, yellow, but all calculated main hue still resided in a rather concentrated blue area (dominant wavelength:476-478 nm) in the CIE chromaticity diagram.However, dominant wavelength of the green fluorescence falls between 473-510 nm with a wide span.