This paper follows International Standards Organization (ISO) by “Guide to the Expression of Uncertainty in Measurement, GUM”. In order to measure Ultraviolet Index (UVI) standard, it is necessary in calibration or test laboratory field norm on the authentication. By distance Inverse-Square Law of the Optical Radiation, We constructed measurement equation, to consider the error source when you are measuring, to evaluate the uncertainty of system. Evaluate the uncertainty of this system: The measurement distance is 75mm, ultraviolet index of the standard source is y = 11.7 UVI, effective degrees of freedom is Veff = 67, coverage factor is k = 2.0, confidence level is 95%, the standard uncertainty is uc= 0.13 UVI, expanded uncertainty is U= 0.3 UVI, the result is Y= y ± U= 11.7 ± 0.3 UVI.