Optical absorption spectra for a series of GaMnN samples with different Mn content grown by metal-organic chemical vapor deposition have been investigated. Compared with GaN, the absorption coefficient of Ga1-xMnxN increases in the UV region and an absorption peak can be observed near 144 eV. In addition, the absorption coefficient increases with the increase of the Mn content. The experimental results are in excellent agreement with theoretical data based on density functional theory. Associated with theoretical data, the absorption peak near 144 eV is assigned to 5T2→5E internal transition between the e state and the t2 state of Mn3+ ion.