Abstract: ABSTRACT In the study of qinghaosu against malaria, a formula for estimating half-life of the drug may be derived from two mathematical expressions based on the linear relationship of the dose-effect and the time-effect. The formula ist_1/2(ED)=-0. 3010b_d/b_t.where t_1/2(ED) is half-life of effective dose,b_d and b_t are the slopes of the dose-effect and the time-effect curves, respectively., The effective dose-means the total dose of the unchanged drug and its metabolite(s) which is eqivalent to the original drug. On certain conditions, the formula may also be used to estimate the half-life of other drugs. In this paper the relationship between the half-life of effective doseand the half-life of plasma concentration of drug was discussed, and the application value of the formula was also assessed.