Abstract: Abstract: It is an organic matter and carbonate carbon isotopic composition study on surface lake sediments from lakes in Songnen Plain, northeast of China. To identify the source of organic matter and sedimentary environment in lakes, we analyzed the n-alkanes carbon distribution characteristics of the organic matter in the lake sediments. With the analysis of the lacustrine water medium and sediment characteristics, we obtained the influencing factors of isotopic composition in sedimentary organic matter and carbonate. The results show that different types of organic matter have a great effect on the carbon isotopic composition. δ13C of carbonate become lighter by biologic organic matter, whereas it will be a little heavier when organic matter comes from oil pollution.