The organic carbon isotope (OCI) of lake sediment has often been used to identify the source of organic matter in sediments or watershed vegetation, and gradually becomes a routine proxy indicator. However, when organic matter content changes significantly and in different forms, the OCI results from the unified approach (bulk sample or a graded composition) will produce deviation, no studies have evaluated this problem so far. This research chose sediment samples from rump lake in arid areas, analyzed the organic content and isotope using bulk sample and fine-grained components (screened through mesh size of 120 and 360), respectively. The influence on OCI results based on different sediment grain sizes and with different organic matter forms was evaluated. Results reveal different OCI values in those sediment even with similar organic matter from different sediment components. Obviously, this relates to the forms of organic matter. It shows that fine-grained components (screened through mesh size of 360) is more advantageous to full reaction, and obtain a more stable value. When comparing the δ¹³C_org value in different lake sediments, researchers should adapt same methods for sample pretreatment. The result has an important implication for the research on organic carbon isotope in lake sediments.