Kelussia odoratissima Mozaff. is a species of Apiaceae endemic to the Zagros Mountains in Iran. In the present investigation, for the first time, the polyphenolic compounds and flavonoids of its leaves were determined by liquid chromatography-mass spectrometry (LC-MS). As a result, p-coumaric acid, ferulic acid, caffeic acid, chlorogenic acid, acetyl phloroglucinol, vanillic acid, m-coumaric acid, and 4-methylsiringol were determined as the main phenolic compounds, while 3-hydroxyflavone, flavone, quercetin, rutin, neohesperidin, polydatin, and diosmin were the main flavonoid components, of which chlorogenic acid (303.08 µL/gDW), neohesperidin (38.37 µL/gDw), and diosmin (28.62 µL/gDW) were the most abundant. Solid-phase microextraction (SPME) was also used to determine the chemical compounds. Based on SPME, (Z)-undec-6-en-2-one (17.48%) and (Z)-butylidenephthalide (4.348%) were the major components. Based on GC-MS analyses, (Z)-ligustilide was the main compound; however, some new compounds were also determined, including 3-ethylisobenzofuran-1 (3H)-one, (E)-ligugustilide, and E-n-butylidene phthalide. Also, for the first time, we have identified EOs ethyl and isobutyl phthalides on the basis of the obtained EI-MS spectra. Finally, the fragmentation of phthalides is also discussed in this research.