Acaricidal activities of the extracts from Inula britannica against Tetranychus cinnabarinus and theri effects on several enzymes in T. cinnabarinus were evaluated under laboratory conditions. It was found that crude extracts from I. britannica with petroleum ether exhibited high acaricidal activities against T. cinnabarinus, and the corrected mortality in 24 h after treatment was 92.05% at concentration of 2 mg·mL-1. After a liquid-liquid partition from petroleum ether crude extracts with methanol, the petroleum ether extracts were separated into 38 fractions by column chromatography, and further tests for their acaricidal activities were conducted. Fraction 33, the main component of the resulting extracts, was found to possess the strongest acaricidal activity against T. cinnabarinus in all 38 fractions, and its corrected mortality against T. cinnabarinus was 90.12% in 24 h. Moreover, GC-MS analysis revealed that chemical composition of fraction 33 was ethyl palmitate. Laboratory bioassay indicated that the corrected mortality of ethyl palmitate against T. cinnabarinus after treatment for 24h was 90.61%, and the mean lethal concentration (LC50) was (1.255 ± 0.167) mg·mL-1. In order to investigate mechanism of the toxic effect of ethyl palmitate on T. cinnabarinus, several important enzymes including glutathione S-transferase (GST), acetylcholinesterase (AChE) and Na+-adenosine triphosphatase (Na+-ATPase) as well as total protein content in T. cinnabarinus were tested by the colorimetric method. The experimental results showed that after treatment with ethyl palmitate, total protein content in T. cinnabarinus obviously increased, and the activities of GSTs and AchE in T. cinnabarinus were strongly induced, while the activity of Na+-ATPase in T. cinnabarinus was inhibited, which could block the transmit of nerve, and eventually result in the death of the mite. These results indicated that Inula britannica extracts possessed high acaricidal activity.