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Dissolved inorganic carbon isotope (d13CDIC) is an important tool to reveal the carbon cycle in lake systems. However, there are only few studies focusing on the spatial variation of d13CDIC of closed lakes. Here we analyze the characteristics of d13CDIC of 24 sampled lakes (mainly closed lakes) across the Qiangtang Plateau (QTP) and identify the driving factors for its spatial variation. The d13CDIC value of these observed lakes varies in the range of - 15.0 to 3.2%, with an average value of - 1.2 parts per thousand. The d13CDIC value of closed lakes is close to the atmospheric isotopic equilibrium value, much higher than that in rivers and freshwater lakes reported before. The high d13CDIC value of closed lakes is mainly attributed to the significant contribution of carbonate weathering in the catchment and the evasion of dissolved CO2 induced by the strong evaporation of lake water. The d13CDIC value of closed lakes has a logarithmic correlation with water chemistry (TDS, DIC and pCO2), also suggesting that the evapo-concentration of lake water can influence the d13CDIC value. The d13CDIC value shows two opposite logarithmic correlations with lake size depending on the d13CDIC range. This study suggests that the d13C in carbonates in lacustrine sediments can be taken as an indicator of lake volume variation in closed lakes on QTP. Copyright (c) 2011 John Wiley & Sons, Ltd.