Geometrical structures for the (NO) 2 are calculated using ab initio Hartree-Fock (SCF), second-order Moller-Plesset perturbation, and coupled cluster with the single, double, and triple substitution methods with a triple zeta plus polarization basis set including diffuse Rydberg basis functions. The structure of (NO) 2 can be described by two interactions (N...N, N...O). One is the ONNO structure with an (N...N) interaction. In this structure, acyclic cis-ONNO with C 2 v -symmetry, acyclic trans-ONNO with C 2 h , and cyclic ONNO with trapezoidal structure (C 2 v ) are optimized at the MP2 level. The other structure is the ONON structure with an (N...O) interaction. In the structure, acyclic cis-ONON with C s -symmetry and cyclic ONON of the rectangular (C 2 h ), square (D 2 h ), rhombic (D 2 h ), and parallelogramic (D 2 h ) geometries are also optimized. It is found that acyclic cis-ONNO ( 1 A 1 ) is the most stable structure. The binding energies and the relative energy gaps among the isomers are found to be relatively small.