Abstract <p>The analysis of a spectrum of ¹⁵N¹⁸O molecule recorded in the region 5100–5500 cm⁻¹ was performed, and 187 positions of rotational lines were found in the vibrational band 3–0 of the main transitions between the electronic states ²Π_1/2 and ²Π_3/2 with the maximal rotational quantum number <i>J</i> = 32.5. For the first time, Λ-splitting was observed in this band. The positions and relative intensities of both the resolved component of Λ-doublets and unresolved doublets were determined. A joint weighted processing of all known vibrational-rotational transition frequencies in the microwave and infrared spectral regions was carried out. As a result, Dunham type constants were determined for ¹⁵N¹⁸O isotopologue in the ground electronic state. Using the found constants, predictive calculations of the rotational line positions of all bands corresponding to vibrational transitions between states with <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({v}\)</EquationSource> <!--OptAtOc2570106Borkov-m1--> </InlineEquation> ≤ 3 and <i>J</i> ≤ 37.5 were carried out. The experimental data obtained in this study provide new spectroscopic information about ¹⁵N¹⁸O isotopologue and enrich fundamental knowledge about nitrogen monoxide molecule.</p>