Each medicinal herb contains one or more effective materials that are defined for therapeutic use according to the active ingredient of the plant. Since chemical drugs have side effects   the value of medicinal plants is still preserved after several centuries. One of these spectroscopic methods for identifying functional groups in medicinal plants is (FTIR). This study aimed to identify the functional groups of Mentha longifolia medicinal plants native to Ilam by Fourier-transform infrared spectroscopy (FTIR) method so that by recognizing the main and effective compounds of plants we can more easily step towards, recognizing and producing biomolecules and herbal medicines. According to the results of spectroscopy, it was found that this plant has 15 spectra, including groups O-H, C-H, C = C, N-O, C-O, S = O, C-Br, and C-I.