Abstract: On the basis of the current situation and theory of diamond colour change domestic and overseas, natural type I diamonds were selected as object of the study, Experiments including irradiation and HPHT treatments were carried out on the 25 natural type laAB diamond samples. The authors compared and analyzed the differences before and after treatment by using infrared spectrometer and UV-Vis spectrophotometer, In irradiation experiment,because of the different irradiation time, the samples received different irradiation dose, but all the samples changed into light blue, bluish green or green. After multi-process treatments including irradiation and HPHT, diamond samples turned into golden yellow. The infrared absorption spectra of diamonds before and after irradiation do not show much change.Some peaks may have a little change in intensity, but the type of diamonds does not change.UV-Vis spectra of samples after irradiation treatment show GR radiation damage at 741 nm,and the transmittance among 550-750 nm weakened, while the transmittance among 750-800 nm enhanced, After HPHT, the peak of UV-Vis spectra at 741 nm disappeared,and the transmittance during 500-800 nm increased, and as a result the treated diamonds appear brighter than untreated ones.