The components and degree of the uncertainty of determination of dihydroartemisinin in Dihydroartemisinin and Piperaquine Phosphate tablets by HPLC are analyzed.The degree of synthesizing uncertainty and the expanded uncertainty under all conditions for component determination was calculated by the analysis and assessment through uncertain compoents.The expanded uncertainty was U=0.8mg(k=2).The first cause for the uncertainty of measurement was the precision of peak areas of the sample solution of Dibydroartemisinin.