Abstract: Quartz in sediments from the Yangtze and Yellow Rivers was separated by potassium hydrogen sulfate (KHSO₄) fusing-hydrofluorosilic acid (H₂SiF₆). Oxygen was extracted from the quartz fraction by BrF₅ (used as oxidant), and then its isotopic composition was analyzed by mass-spectrometer, expressed as δ¹⁸O (¹⁸O/¹⁶O)values (relative to SMOW). The results show that δ¹⁸O values generally increase with grain size of quartz changing from coarse to fine. The oxygen isotope ratios (δ¹⁸O) of quartz from the Yangtze River are higher than those from the Yellow River. The differences in δ¹⁸O values of the same grain-size quartz between the Yangtze and Yellow Rivers may have resulted from different sediment sources and climate conditions along these two river basins.