The crystal structure of a lutetium silicon oxynitride (Lu4Si2O7N2) was analyzed by the Rietveld method using time-of-flight (TOF) neutron powder diffraction data. The compound crystallizes in a monoclinic cell, space group P2(1)/c (No. 14-1) with a = 7.4243(1), b = 10.2728(1), c = 10.6628(1), Angstrom and beta = 109.773(1)degrees at 297 K. One nitrogen atom in Lu4Si2O7N2 occupies the bridging site between the two S! atoms, and the other one is statistically situated at the terminal sites of Si2O5N2 ditetrahedra. In the local structure, Si2O5N2 ditetrahedra consist of SiO3 and SiO2N2 tetrahedral units sharing the N atom. Lu atoms are in sixfold, sevenfold (x2) and eightfold coordinations of O/N atoms. X-ray powder diffraction data were also analyzed with the model obtained by the neutron diffraction.