On the basis of different photosynthetic pathway, there's obvious difference in delta 13 C values between C3 plants and C4 plants. Use this characteristic, the delta 13C values in different size and density fractions of two profile soil samples either in farm land and forest lands near Maolan Karst virgin forest was analyzed, there plant C3 plants previously and plant C4 plants now. Results showed that the delta 13C values of different size fractions in forest soil aere delta 13Ccoarse sand < delta 13Cfine sand < delta 13Ccoarse silt < delta 13Cclay < delta 13Cfine silt, and the delta 13C values of different size fractions in farmland soil were delta 13Ccoarse sand > delta 13Cfine sand > delta 13Ccoarse silt > delta 13Cclay > delta 13Cfine silt, it indicated that the soil organic matter was fresh in coarse sand and oldest in fine silt. The delta 13C values of different density fractions in forest soil were delta 13Clight < delta 13Cheavy, and the delta 13C values of different density fractions in farmland soil were delta 13Clight > delta 13Cheavy, it also indicated that the soil organic matter was fresh in light fractions and old in heavy fractions.