Abstract: In drinking water treatment, prechlorination and preozonation are often applied in order to control microorganisms and taste-and-odor-causing materials, which may influence organics removal by preoxidation and adsorption. Using commercial and natural water humid substances, the positive effect of prechlorination and preozonation as an aid to coagulation-flocculation of these compounds were confirmed by removal of TOC removal in Tehran city (capital of Iran). These experiments were conducted at bench-scale through a series of jar test using different pH, coagulant dosage and total organic carbon concentration of approximately 4, 8 and 12 mg L-1. In addition to TOC removal the existence of an Optimum Preozonation Dose (OPZD) and an Optimum Prechlorination Dose (OPCD) were also confirmed. Experiments show that prechlorination and preozonation can improve of coagulation and flocculation depended on TOC concentration of influent surface raw water. The results demonstrate differential effect between prechlorination and preozonation on removal of TOC. Preozonation showed a positive effect on a system with low influent TOC and very low molecular weight (noncolloidal) humic substances.