Abstract: Naphthenate is a kind of monocarboxylic acid with carbon ring structure, and, it consists in crude oil as monocarboxylic acid that has different molecular weight. The caustic-treated effluent from the cut fraction in the crude oil is treated by acid, then, industrial naphthenate is prepared. The purity of naphthenate in the industrial naphthenate is a most important standard used to estimate the property of naphthenate. In the graph by infrared spectrometry determination method, the carboxyl exists in the 1 700 cm^-1 with significant high peak. That exclusive peak won’t be interfered with the other groups in the sample. Therefore, the infrared spectrometry method is perfect to determine the purity of naphthenate in industrial naphthenate.