In order to improve the accuracy in measurement, the uncertainty of determination of Ca in Areca by atomic absorption spectrophotometer (AAS) was evaluated according to “JJF 1059.1—2012 Evaluation and Expression of Uncertainty in Measurement”. First, a mathematical model of determination of Ca in Areca was established at first. Then, the source of uncertainty was analyzed and the uncertainty components were quantified. Finally, the synthetic uncertainty and expanded uncertainty were calculated. The uncertainties of standard curve fitting and AAS had a greater influence on the expanded uncertainty. When the content of Ca in Areca was 3.16 mg/kg, the expanded uncertainty was 0.04 mg/kg (k=2, P=95%).