Abstract: Fatty acids in plant oil are esterified by thermal tetramethyl ammonium fatty acidate and determined by gas chromatography.The result shows that:When pH=7.0,fatty acid content in plant oil esterified with tetramethyl ammonium fatty acidate can be determined accurately by gas chromatography.The reqular method that starts with saponification and than followed by methyl-esterrification is not applicable to measure linoleic caid and linolenic acid.About 10%~20% of erucic acid is contained in the seeds of all species of genus Acer.