Abstract: A detailed discussion is given in the paper with regard to the mathematical treatment and relevantnumerical techniques of uncertainty analysis used in FRAP codes (FUA). Following the discussion, animproved uncertainty analysis method (IFUA) is proposed. In IFUA, all basic mathematical treatmentsused in FUA remain unchanged, meanwhile three alternative procedures are introduced into IFUA.They are: (1) Latin hypercube sampling; (2) stepwise regression; (3) probabilistic density functionfitting using the Pearson curve family. A sample problem demonstrating the effectiveness of IFUA isaccomplished. The results show that the IFUA improves the capability of FUA and is more convenientand effective for practical use.