Tetrandrine is a benzylisoquinoline alkaloid that has been used in China as an antifibrotic drug to treat the lesions of silicosis. Its mechanism in the treatment of silicosis is unclear. Electron spin resonance (ESR) spin trapping was employed to investigate the antioxidant properties of tetrandrine. The spin trap used was 5,5‐dimethyl‐1 ‐pyrroline N‐oxide (DMPO). Tetrandrine efficiently reacted with hydroxyl COH) radicals with a reaction rate of approximately 1.4 × 1010M−1s−1The *OH radicals were generated by the Fenton reaction [Fe(II) + H2O2) as well as by reaction of chromium(V) with H2O2. Similar results were obtained using *OH radicals generated by reaction of freshly fractured quartz particles with aqueous medium. Tetrandrine also scavenged superoxide (O2−) radicals produced from xanthine/ xanthine oxidase. The effect of tetrandrine on lipid peroxidation induced by freshly fractured quartz particles was evaluated using linoleic acid as a model lipid. The results showed that tetrandrine caused a significant inhibition on freshly fractured quartz‐induced lipid peroxidation.