Abstract: Free formaldehyde in cosmetic was determined by HPLC combined with automatic online derivatization. Free formaldehyde in cosmetic samples was extracted by acetonitrile and reacted efficiently with 2,4-dinitrophenylhydrazine in acetonitrile-acetic acid solution for 2 min. The formaldehyde derivative was separated on an Agilent C₁₈ chromatographic column with acetonitrile-water (65+35) as the mobile phase for elution, and then detected by DAD at the wavelength of 355 nm. The linear range of formaldehyde was 1.0-20.0 mg·L^-1, with the lower limit of determination (10S/N) of 50 mg·kg^-1. Recovery rates obtained by standard addition method were in the range of 83.0%-98.4% and RSDs (n=6) were in the range of 3.4%-7.8%.