Cordycepin was extracted and purified from wheat medium of Cordyceps militaris and its inhibitory effect on Hep G2 cells was studied in vitro.The extraction of cordycepin was optimized using a thermosonic water bath at varying pH values, temperatures, extraction times and ultrasonic power.The results show that the extraction was better when solid to liquid ratio of 1∶50,at pH 6,at of 60 ℃,with an extraction duration of 3 hours,and 300 W ultrasonic power.The purity of cordycepin was over 99%.The extracted cordycepin was then purified employing a macroporous adsorptive resin (AB-8),and was identified by HPLC and Infrared spectroscopy.The growth of Hep G2 cells treated with the purified cordycepin was measured using the MTT assay,and the morphological changes of Hep G2 cells were observed by means of inverted differential microscopy.Different concentrations of cordycepin showed significant inhibitory effect on Hep G2 cells,and showed a positive correlation with concentration and inhibitory time.The inhibited cells became round,agglutinated and fragmented.