In order to advance the international exchange and the development, the International Organization for Standardization (ISO) provides the Guide to the expression of uncertainty in measurement (GUM). According to ISO GUM, CNS 2339 Method of test for mixing ration of fibers mixtures and AATCC 20A Fiber analysis: quantitative, this article is to take one blended fabric of wool and polyester fibers as an example (the ratio of wool to polyester fibers is approximately 60:40), to analyze, and to evaluate the uncertainty in quantitative analysis of fiber mixtures.