Abstract: A method of HPLC for the determination of lambda cyhalthrin acid in pesticide was proposed. The sample was extracted ultrasonically with methanol. The extract obtained was separated by using Agilent Eclipse XDB-C18 chromatographic column as stationary phase, and mixture of methanol and (φ) 0.5% phosphoric acid solution (55+45) as mobile phase. UV-detection at 230 nm was adopted in the determination. Linear relationship between values of peak area and mass concentration of lambda cyhalthrin acid was kept in the range of 0.025-0.5 g·L-1, with detection limit (3S/N) of 0.112 mg·L-1. Values of recovery found by standard addition method were in the range of 97.5%-98.1%, and values of RSD′s (n=6) less than 2%.