Abstract: In order to prevent flue-cured tobacco leaf from darkening in the course of curing and processing,the experiments on inhibiting polyphenol oxidase(PPO) activity in flue-cured tobacco with citric acid,boric acid,sodium diethyl disulphocarbamate(NaDDC),ethylene diamino tetracetic acid(EDTA),polyvinyl pyrrolidone(PVP),l-cysteine,ascorbic acid(Vc),and thiourea in phosphate buffer solution were conducted.The results showed that:1) the PPO activity in phosphate buffer solution was effectively inhibited by PVP(1.0 mmol/L),l-cysteine(0.6 mmol/L),Vc(1.0 mmol/L) or thiourea(3.2 mmol/L);2) the mixture of PVP(1.0 mmol/L)+thiourea(3.0 mmol/L)+Vc(0.75 mmol/L)+l-cysteine(0.375 mmol/L) or the mixture of thiourea,Vc and l-cysteine was more effective.