Abstract: Hogwash oil is the poor quality illegal cooking oil. Illegal or adulterated oils are harmful to the human health. A simple and effective derivatized with 2, 4-dinitrophenylhydrazine method had been developed for sensitive determination aliphatic monoaldehydes, including formaldehyde, acetaldehyde, propanal, butanal, pentanal and hexanal in various oil samples to identify the illegal oil by HPLC. The result showed that the proposed method provided a reliable and sensitive quantitative evaluation for illegal cooking oil sample matrices, moreover, provided technical support for the supervision and administration of the edible oils.