Reactions between chlorine disinfectants, total organic carbon (TOC), and other chemicals in water form a series of disinfection by-products (DBPs), including trihalomethanes (THMs) that are toxic and suspected carcinogens. This paper aims at reducing DBPs formation potential through the degradation of the precursors using pre-oxidation. First, a survey analysis for five different water treatment plants (WTPs) was carried out to determine the most polluted site with TOC and THMs afterward, the efficiency of three different oxidants (potassium peroxy-mono-sulfate, potassium permanganate, and hydrogen peroxide) was tested under different conditions of pH, oxidant concentration and contact time. Optimum values for the mentioned conditions were used to achieve the maximum reduction for THMs in Mostorod-WTP which recorded the highest concentration of TOC and THM. THMs reduction efficiency of 54%, 67%, and 99% was achieved by using potassium peroxy-mono-sulfate, potassium permanganate, and hydrogen peroxide, respectively. In conclusion, hydrogen peroxide was the best in reducing THMs.