After HPHT treatment using the cubic-anvil press, the brown diamond was investigated by spectral analysis, followed by color changing mechanism analysis. The results show that brown diamond including ⅠaA and IaAB becomes yellow 30 minutes later under the condition of 5.6 GPa and 1 650℃, which is mainly related to H3, H4, N3 centers and single nitrogen; then it will become yellowish green after HPHT treatment, which is related to H2, H3 centers.