The potential energy surface (PES) of nitrobenzene isomers has been built by CCSD(T)/CBS methods. The geometries of isomers and transition states (TSs) are optimized at BHANDHLYP/6-311 + + G (d, p) level. Sixteen isomers and fifteen isomerization reactions of nitrobenzene were found. The rate constants for the isomerization reactions are calculated and compared according to different types of isomerization. The rate constants and activation energies (Ea) of all reactions are fitted using a qusi-Arrhenius equation. Branching ratios of the nitrobenzene isomerization reactions and the previously reported decomposition reactions are compared and the isomerization to C6H5O-NO followed by cleavage of O-NO dominates the decomposition of nitrobenzene at low temperature.