The absorption spectrum of NO is analyzed in a range of 12530-12850 cm-1 using optical heterodyne concentration modulation spectroscopy in an AC glow discharge of He/NO mixture. Among all the spectral lines, 324 lines are assigned to the (4, 0) band and 267 ones are fitted to the Hamiltonians of the b4Σ--a4Πi system by a non-linear least-square fitting procedure. A residual (0.0071 cm-1) comparable to system error (0.0071 cm-1) is obtained. The main molecular constants are consistent with those reported in the literature and the fine structure constants are fitted.