||High Pressure High Temperature (HPHT) treatment can significantly change the color of diamonds. In this study, we studied optical properties variation according as nitrogen arrangement of various types in natural brown diamonds (type IaAB, type IaB, type IaA ＞ B, type IaA ＜ B, IaA = B) after HPHT treatment. The diamonds with deferent arrangements of nitrogen were annealed at temperature in the range 1700 - 1800 ℃ under a stabilizing pressure of 5 GPa. HPHT treated samples were analyzed using UV-Vis-NIR, FT-IR, and PL spectroscopy. Absorption and luminescence spectra have been measured to compare the variations of nitrogen arrangement in natural brown diamonds before and after HPHT treatment. After HPHT treatment, brown coloration in diamonds of all type was reduced. Furthermore, spectroscopic analysis of the natural brown diamonds after HPHT treatment showed that yellow color was produced by absorption in the H3 centers and the green color was a result of being combined absorption in the H3 and H2 centers.