This research attempted to obtain the optimum addition amounts of TiO2 and H2O2 and appropriate pH condition for wastewater treatment using photocatalysis of TiO2. Furthermore, this wastewater treatment technique was also applie to the extreme cases where main rw materials and chemical additives could cause very high NBDCOD for measuring the efficiency of this technique. The photocatalysis reaction was applied to wastewater after secondary biological treatment in linerboard mill in order to obtain optimum conditions. It turned out that the optimum addition amounts of TiO2 and H2O2 were 2g/L and 200mg/L, respectively. The optimum pH was 3.0. In case of the raw materials the treatment of KOCC and AOCC degraded COD for 4 hours by 94.4% and 97.3%, respectively. In case of chemical additives, the treatment of fluorescent brightening agent and direct black dye degraded COD for 12 hours by 89.1% and 93.7%, and the treatment of SB Latex degraded COD for 1 hours by 98.9%.