A novel ionic liquid based ultrasonic assisted extraction（ILUAE）combined with reverse phase high performance liquid chromatography tandem UV detection（HPLC-UV） was first developed to isolate and determinate flavonoids in rhododendron mucronatum leaves.The 1-butyl-3-methylimidazolium chloride（［BMIM］Br） aqueous solution was selected as extraction solvent.The sample was separated on a Phenomenex C18 column using acetonitrile-0.25% acetic acid as mobile phase by gradient elution at a flow rate of 1.0 mL?min-1.The UV detection wavelength was set at 355 nm.The quantitative analysis of rutin，quercetin and kaempferol were carried out by the external standard method.Under the optimal extraction conditions，the calibration curves of rutin，quercetin，and kaempferol were linear in the ranges of 0.064-128，0.076-152 and 0.064-128 mg?L-1，respectively，with correlation coefficients of 0.999 9.Their limits of detection（LODs，S/N=3） were 0.006 6，0.009 2 and 0.005 0 mg?L-1，respectively.The spiked recoveries of three compounds ranged from 90% to 97%.The results indicated that the proposed method was an environment-friendly，easy to operation and efficient technique for sample preparation，thus it could provide an alternative means for assaying flavonoids in the rhododendron mucronatum leaves.