The sedimentation rate and fractional dimension of the floc was studied during the process of the organic waste water treatment with modified natural polymer (CMS) and polymeric aluminum chloride (PAC). Irregular garrulous form of the floc was observed with microscope and electron-scanning microscope. The results showed that the sedimentation rate was 2.6 mm/min with the modified natural polymer (CMS) and 1.7 mm/min without CMS in condition of keeping the pH value to 6.86 and added quantity of PAC to 80 mg/L. The fractional dimension of the floc Df was calculated to be 1.70 in the best sedimentation rate.