Conflicting results in the effect of OC on Atterberg limits need to a clarification with more studies. The observation that why PL increases in the higher rate than LL would be interesting issue to more investigation. Thus, this paper is focused on the index properties of OS (Site A) and SOS (Site B). It was found that removing OC in Site A affect particle size distribution by a dispersion phenomena while no effect was observed in Site B. Presence of average 13.6 % OC in site A leaded to increase in liquid and plastic limits for 6.1% and 26.9 %, respectively.