The accumulated doses of samples are calculated by nonlinear fitting of the ESR dose response curves. The method of calculating AD’s uncertainty has been improved on the basis of Grun’s work. The problems and limitations for the linear fitting usually used are analyzed and the requirements for data in the non-linear fitting are discussed. The uncertainty of ESR ages of the first skull of Peking-Man and deposit at Zhoukoudian (Loc.l) is given.