Oxidized lipids can decrease the absorbance of DPPH in isooctane. Changes of aldehydes, which can react with DPPH radicals, in oxidized linoleic acid and lard were monitored using 2,4-dinitrophenylhydrazine (DNPH) derivatization and HPLC. As thermal oxidation time increased in linoleic acid, peaks of propanal, pentanal, and hexanal increased proportionally. The correlation coefficient (r) of among DPPH absorbance and peak areas of DNPH derivatized lard for 360 min was −0.941. Peaks of t-2-heptenal and pentanal showed relatively high correlation with the changes of DPPH absorbance during lard oxidation. Quantity of oxidized products with radical scavenging ability in lipids can be determined by DNPH derivatization with HPLC analysis.