Abstract: Isolated nitrogen defects represent a significant colour-causing factors in natural yellow diamonds, which exhibit significant diversity in practical detection. This study systematically classifies 91 yellow diamond samples containing isolated nitrogen defects, and explores their spectral characteristics. The infrared absorption spectra(1 344 cm^-1 and/or 2 688 cm^-1), coupled with UV-Vis absorption spectra (400—560 nm continuous absorption) confirmed isolated nitrogen defects as the colouration of yellow diamond samples. By calculating normalizing the data of infrared absorption spectra of total nitrogen concentrations and aggregation degree of C-defects, A-defects, and B-defects, the yellow diamond samples were categorized into 4 types: pure type Ⅰb, typeⅠaA—Ⅰb, typeⅠb+ⅠaAB (with unsaturated nitrogen absorption in the IR spectrum) and type Ⅰb+ⅠaAB (with saturated nitrogen in the IR spectrum), with increasing total nitrogen content and nitrogen aggregation degree. The infrared absorption spectra of pure type Ⅰb, type ⅠaA—Ⅰb, and type Ⅰb+ⅠaAB (with unsaturated nitrogen) diamond samples exhibite the absorption peaks related to the amber center, absent in type Ⅰb+ⅠaAB (with saturated nitrogen). Additionally, the luminescence images and photoluminescence spectra revealed that the first three types exhibit obvious slip line structures caused by plastic deformation, with characteristic PL peaks at 503.2, 637.2, 828, 831, 905 nm.The fourth type shows fluorescence structure similar to "zoning" without slip line structures, with characteristic PL peaks at 496.2, 701, 787, 926 nm.