Medium consistency ozone bleaching plant was started up at Yufutsu mill in January 2001. This is the first plant that use ozone gas as a bleaching sequence in Japan. By changing conventional C/D-E/ op-D into ZD-E/op-D, we have entirely excluded the usage of elemental chlorine. The ozone generator is the maximum class in the world as one unit capacity, which can provide a high concentration of ozone in oxygen (12% by weight) at a designed output of 120 kg of ozone/hr.The pulp that is bleached by the ozone ECF sequence has not had any troubles with a quality of the paper products. The cost of ozone ECF is lower than the cost of ECF that was made from chlorine dioxide as a main bleaching chemical. The results of the ozone ECF is as good as was expected from the laboratory studies. Effluent AOX and chloroform emissions have decreased significantly for starting up the ozone bleaching plant.