Abstract: Conversion of native land, into a cultivated system causes precipitous degradation of the soil organic matter. It is reported that C stored in the clay mineral fraction contributes more to the long-term stability of C than sand and silt fraction. This study reports on the soil organic carbon in the clay mineral fraction of some cultivated lands in Sri Lanka and how it deviates from the adjacent natural forests. The field sites included 7 cultivated lands (tea, rubber, coconut, mixed crops, potato, home garden, chena cultivation) and 7 tropical forest types (wet evergreen, semi evergreen, moist monsoon, dry monsoon, montane, dry mixed evergreen). The significant differences in carbon content of the clay fraction between the cultivated land and the adjacent forest in the same location revealed that cultivation has decreased the carbon content in all sites. The highest C content shown by the mixed crop and rubber plantations showed that minimum land management and soil tillage involved with this reduced the rate of decomposition. High temperature of dry mixed evergreen forest and dry monsoon forest may have increased the decomposition rates and lowered the SOC in the clay fraction compared to the other forests. Significant correlations were observed in forests with in clay fraction and climatic parameters but to a lesser extent in cultivated lands. Considerable variations in organic matter input in cultivated lands will be the reason for these weak relationships. The study confirmed that clay mineral fraction was also equally affected during long term cultivation although it was reported to be more stable against rapid decomposition.