The final aim of biological wastewater treatment is removal of dissolved organic matters. Sludge bulking is one of the most common problems relateing to activated sludge wastewater treatment, and this causes poor settling of sludge in secondary sedimentation tank. Although very extended researches have done on the sludge bulking, this problem hasn’t eliminated completely yet. The natural additives are materials that exist in the environmen, and frequently enter the to the environment as solid wastes. Assessment of probable influence of natural waste materials on the wastewater settling properties was the principal aim of this research. Sludge settling problems exist in most wastewater treatment plants in Iran and maybe using natural additives can be one of the solutions for better sludge settling. SVI is one of the operating parameters that used for secondary sedimentation tanks design and control. A SVI value equal to 100 mL/g is typically associated with good settling sludge. The ZSV is the settling velocity of the sludge/water interface (Vi) at the beginning of the sludge settleability test. Second phase of Shahinshahr Wastewater Treatment Plant was selected for wastewater sampling and conducting experiments of this research. All experiments were done with three iterations. Experiments in this research conducted in four separated stages: 1) determination of optimum dosage for different natural additives, 2) determination of zone settling velocity in pilot scale, 3) determination of influence of sludge return on activated sludge settling properties and also determination of some quality parameters in supernatan, 4) determination of influence of different mixing velocities and times on SVI values. Natural additives used in this research includes: maize stalk, almond shell, activated carbon, coconut shell, walnut shell, hazelnut shell, egg shell, peanut hull, silt and clay and pine bark. SVI tests were done in a 1-Lit graduated cylinders. Settling column used for determining of ZSV was a 200 cm high plexiglass cylender (190 cm of this column was filled with wastewater samples), 20 cm diameter and 60 L volume. Average values of SVI for control sample was 823 mL/g. Optimum dosage for different natural additives is defined as that reduced SVI to 100 mL/g (88 percent decrease). Average values of ZSV for control sample was 1.222 m/h. Different natural additives increased ZSV of control sample to 2 to 3 times. This significant in ZSV is important, because as a consequence of that the surface overflow rate (OFR) will increase 2 to 3 times too. In this research, influence of sludge return on activated sludge settleability was also evaluated. It was observed that this caused an improvement on sludge settleability characteristics. Natural additives act as ballasting agents therefore, this mechanism is the main responsible of decreasing SVI value of sludge.