Fatty acid compositions of the seed oils of P. schinseng, A. continentalis and A. sessiliflorus, were analyzed by gas chromatography (GC) equipped with a capillary column. A large unusual peak was observed just before the peak corresponding to oleic acid $(cis-9-C_{18:1})$. This unknown fatty acid was isolated by silver ion chromatography and then derivatized into the picolinyl ester. The mass spectrum of the picolinyl ester showed molecular ion at m/z=373 with other diagnostic ions such as m/z=178, 218, 232, 246, 274, 288, 302 and 344. Characteristic absorption peaks at $720;cm^{-1}$, $1640;cm^{-1}$ and $3010;cm^{-1}$ in IR spectrum indicated the presence of cis-configurational double bond in the molecule. The $^1H-NMR$ spectrum of this acid gave two quintets centered at ${delta}1.638$ (2H, C-3) and ${delta}1.377$ (2H, C-4), and two multiplets centered at ${delta}2.022{sim}2.047$ (2H, C-5) and ${delta}2.000{sim}2.022$ (2H, C-8), and multiplet signals of olefinic protons centered at ${delta}5.3015{sim}5.3426$ (C-6, J=9.5 Hz) and ${delta};5.3465{sim}5.3877$ (C-7, J=9.5 Hz). The $^13C-NMR$ spectrum showed 18 carbon resonance signals including an overlapped signal at ${delta}29.7002$ for C-12 and ${delta}29.6520$ for C-13 (or they can be reversed), and other highly resolved signals at ${delta}33.950$, ${delta}24.558$, ${delta}26.773$ and ${delta}27.205$ due to C-2, C-3, C-5 and C-8 of a ${Delta}^6-octadecenoic$ acid, respectively. From analysis results this unknown fatty acid could be identified as cis-6-octadecenoic acid. The seed oils of P. schinseng and A. sessiliflorus contained petroselinic acid (59.7%, 56.0%), oleic acid (18.3%, 6.1%) and linoleic acid (16.2%, 30.4%) with small amount of palmitic acid (3.0%, 3.1%) while the seed oil of A. continentalis comprised mainly oleic acid (30.2%), petroselinic acid (29.0%), linoleic acid (24.1%) and palmitic acid (13.1%).