The rotational spectrum of the NI radical in the X (3)Sigma(-) ground electronic state has been observed by a source modulated spectrometer with a free space cell. The NI radical was generated in the cell by a dc (direct current) glow discharge in a mixture of iodine vapor with N-2 and He. A rotational transition consisted of a group of congested lines resulting from the hyperfine interactions due to both iodine and nitrogen nuclei. The molecular constants including the spin-spin and spin-rotation coupling constants and the magnetic and nuclear quadrupole hyperfine interaction constants were determined by the least-squares analysis. The spin density of the unpaired electrons was estimated from the observed hyperfine coupling constants to be 76.8% and 23.4% on the nitrogen and the iodine atoms, respectively. The quadrupole coupling constant was related with the difference in electronegativity of the two bonded atoms.