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Organic carbon isotopic composition (δ[sup13]C) is one of the important proxies in paleoenvironment studies. In this paper modern plant δ[sup13]C in the arid areas of China and Tibetan Plateau is studied. It is found that most terrestrial plant species in western China are C[sub3] plants with δ[sup13] C values ranging from -32.6% to -23.2% and only few species are C[sub4] plants with δ[sup13]C values from -16.8% to -13.3%. The δ[sup13]C is closely related to precipitation (or humidity), i.e., light δ[sup13]C is related to high precipitation (or humid climate), while heavy δ[sup13]C to low precipitation (or dry climate), but there is almost no relation between plant δ[sup13]C and temperature. Submerged plants have δ[sup13]C values ranging from -22.0% to -12.7%, like C[sub4] plants, while merged plants have δ[sup13]C values ranging from -28.1% to -24.5%, like C[sub3] plants. It can then be concluded that organic δ[sup13]C variations in terrestrial sediments such as loess and soil in western China can indicate precipitation changes, but those in lake sediments can reflect organic sources and the productivity of different types of aquatic plants. [ABSTRACT FROM AUTHOR]