Abstract: Natural colourful diamonds are rare，and most natural diamonds are with yellow or brown tone.In addition，HTHP synthetic diamonds are more or less with yellow or brown tone，which can only be used in manufacture.If the colour of those diamonds could be turned into attractive colour，their values would be increasing greatly.Nowadays，multi- process treatment is the main method to change the colour of diamonds，and red is one of the most desirable colour that can be obtained. Based on the mechanism of diamond colour change， irradiation treatment and H T HP treatment are carried out on 15 pieces of HTHP synthetic diamonds, and as a result a series of red diamonds are obtained. In irradiation treatment, electron beam is used to irradiate diamond samples, and in HTHP treatment six sides artificial hydraulic machine is used, with the temperature ranging from 1 300℃ to 1 400℃ and pressure set is 100 MPa. Using infrared spectrometer and USB4000 fiber optic spectrometer, infrared spectra and UV-Vis spectra are tested before and after the treatment to study the mechanism of the colour change and identification characteristics. After irradiation and HTHP treatments， the type of diamonds does not change. The absorption at 741 nm in UV-Vis spectrum is the identification of irradiated diamonds. After irradiation and HTHP treatment, intensive absorption at 637 nm can be observed in UV-Vis spectrum, whose intensity is stronger than that of absorption at 575 nm. In HTHP treatment，(N-V)⁰ center. can trap electron and change into (N-V)^-。The absorption between 400 nm and 450 nm and the absorption near 550 nm are also the characteristics of diamonds after irradiation and H THP treatment.