Abstract: The oxygen isotopic composition in organic matter is a powerful tool for studying on biogeochemistry and paleoenvironment. TC/EA-MS technology is a simple and precise method for oxygen isotopic composition. TC/EA-MS technology is used to realize the high precise-on line analysis for the oxygen isotopic composition in organic materials. In this work, the Finnigan TC/EA-Delta plus system is used to determine oxygen isotopic composition of cellulose. The results shows that the precision of measurement of oxygen isotopes is less than 0.05% under the experimental condition of 0.05 MPa for reference gas CO, 0.08 MPa for GC temperature and 0.08～0.10 MPa for carrying gas.