Six isomers of SN2O2 have been found to be minima on the potential energy hypersurface calculated by ab initio and density functional theories. Calculations show that two N-N bond isomers, trans- and cis-S(O)NNO, are close in energy and the predicted vibrational frequencies and isotopic shifts of these two isomers reproduce the experimental doublets satisfactorily, which was referred to trans-S(O)NNO previously. The C-2v-ONSNO is energetically favored and the vibrational frequencies and isotopic shifts are in excellent agreement with experimental observations.