Abstract: Abstract: [Aim] The aim of this study is to determine the acaricidal mechanism of fraction 9 of radix stemonae extracts against T. cinnabarinus. [Method] The glass slide-dipping method was used to determine acaricidal activity of the fraction 9 of radix stemonae extracts against T. cinnabarinus. The effects of the fraction 9 of radix stemonae extracts on four important enzyme activities including AChE, Ca2+-ATPase, SOD and MAO were determined by biochemistry method. [Result] Our studies showed that the fraction 9 of radix stemonae extracts had high toxicity to T. cinnabarinus with a LC50 value of 0.6339 mg?mL-1. After exposure to fraction 9 of radix stemonae extracts, the activity of SOD was activated, while the activities of AChE, Ca2+-ATPase, and MAO in T.cinnabarinus were inhibited at different degrees. The main enzyme activities of the mites were changed by fraction 9 of radix stemonae extracts, which may kill the mites. [Conclusion] The results showed that radix stemonae extracts had high acaricidal activities to T. cinnabarinus, and radix stemonae can be a novel resource of botanical acaricides.