Oxygen/nitrogen (O/N) configuration in RE-J-phase, RE4Si 2O7N2 (RE = rare earth element), was simulated by the Monte Carlo method applied to O/N distribution models. Proportion of local structures of Si2(O5N)7 ditetrahedra and Si(O,N)4 tetrahedra in the J-phase was quantitatively assessed. For the Lu-J-phase model with the bridging site between two Si atoms being occupied by nitrogen atom, the Si2(O,N)7 ditetrahedra composed of O3=Si-N-Si=O2N (> 40%), O3=Si-N- Si≡O3 (c.a. 30 %), and O2N≡Si-N≡SiO 2N(c.a. 15 %). Tetrahedra of SiO3N and SiO 2N2 were dominant and small amount of SiON3 tetrahedra coexisted. For La-J-phase model with the O/N occupancy of 0.1/0.9 at the bridging site, configurations of O3≡Si-O-Si≡O 2N (c.a. 5%), O3≡Si-O-Si≡O3 (c.a. 3%), and O2N=Si-O-Si≡O2N (c.a. 2%) were demonstrated in addition to the three configurations of ditretrahedra recognized in the Lu-J-phase. In La-J-phase coexistence of SiO4 tetrahedra was presented.