Abstract: The measurement uncertainty for the analytical results of H₂SiO₃ in groundwater samples by molybdosilicate yellow spectrophotometry was evaluated using Continuous Propagation Model of Uncertainty. The main sources of measurement uncertainty came from sub-uncertainties of calibration solutions, calibration curve fitting and measurements. The uncertainty from molar masses of SiO₂ and H₂SiO₃ was ignored because of their less contribution. The double-error regression was used in the course of standard curve fitting. Based on the quantitative analysis of each source, standard uncertainty of measurement results was obtained. Then the extended uncertainty of measurement results was calculated from standard uncertainty multiplied by an extended factor of 2 (under confidence probability of 95%).