To analyze the uncertainty evaluation of measurement results for p-hydroxy benzoic acid ethyl ester in edible betel nut by Gas chromatography mass spectrometry, the various factors were found, which affected uncertainty according to JJF 1059.1-2012 Evaluation and Expression Uncertainty in Measurement, and evaluated the synthetic uncertainty and expanded it, gave the uncertainty range of the measured value. It was provided support for methods discussing and evaluation on test reports. It is showed that three kind of factorshave significant contribution to the uncertainty synthesis, such as recovery factor, gas chromatography mass spectrometry and measurement repeatability.