The prechlorination in the drinking water treatment system have produced chlorination by-products such as trihalomethane(THM). For the improvement of the drinking water treatment efficiencies as well as the control of chlorination by-products, this research was performed to find out an adequate pretreatment process for the Nakdong river water using biological aerated filter (BAF) and ozonation as an alternative of prechlorination. For experimental conditions, ozone dosage and contact time of preozonation ranged from 1 to 2 ㎎/L and from 2 to 5 minutes, respectively. The flow rate of the BAF process ranged from 100 to 120 m/day. Based on the results of this research, the removal rates of preozonation and the BAF processes were 23％ and 35％ for KMnO_4 consumed, less than 10％ and 15％ for THM formation potential (THMFP), 40％ and 32％ for color, 41％ and 45％ for ABS, and less than 10％ and more than 90％ for NH_4-N, respectively. Based on above results, the preozonation was effective in reducing color, Fe, and Mn, but not effective on NH_4-N removal. On the other hand, the BAF process was very effective in removing NH_4-N and a little more effective than preozonation in removing ABS, KMnO_4 consumed, and THMFP.