Abstract: HPLC was applied to the determination of benzoyl peroxide in whey powder and butter. Samples, with low fat-content were extracted directly with acetonitrile. After shaking and centrafuging, the supernatant was filtered with 0.45 μm filtering membrane and the filtrate was used for HPLC analysis. Samples, with high fat-content, were first defatted with n-hexane, and then extracted thrice with acetonitrile (which has been saturated with n-hexane). The acetonitrile layers were combined, filtered and used for HPLC analysis. The Agilent TC C18 chromatographic column and the mobile phase of mixture of methanol and water (65+35) were used for the separation. UV-detection at the wavelength of 236 nm was adopted in measurements. Linear relationship between values of peak area and mass concentration of benzoyl peroxide was obtained in the range from 0.1 to 1.0 mg·L-1. Lower limit of determination (10S/N) was found at 1.0 mg·kg-1. Using sample of whey powder and butter as matrixes, recovery was tested by addition of standard solution of benzoyl peroxide at 3 concentration levels (1, 5, 10 mg·kg-1), values of recovery found were in the range of 96.6%-104.0% with RSD′s (n=6) less than 7.0%.