Using high level ab initio molecular orbital methods, theoretical studies were performed on the low-lying excited states of CH3OBr. The complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) methods, with the cc-pVTZ basis set, were used to calculate the vertical excitation energies for the lowest three (1)A', two (1)A'', two (3)A' and two (3)A'' states. The CASSCF method was used to obtain the potential energy curves for the lowest three (1)A', two (1)A'', two (3)A' and two (3)A'' states of CH3OBr along the O-Br coordinate. All of the eight excited states were found to be strongly repulsive along the O-Br coordinate.