Abstract: The measurement uncertainty of silicon determination results for titanite samples by silicomolybdic blue spectrophometry was evaluated and the mathematical model of uncertainty expression was also established. The main sources of measurement uncertainty came from the sub-uncertainties of standard reference materials, sample preparation, calibration curve fitting and repetitive experiments. When the average concentration of SiO2 in samples was 0.67%， the expanded uncertainty was 0.05% (k=2).