To provide a method for bleaching a chemical pulp containing at least one treating stage using chlorine dioxide capable of protecting environment and providing a strongly bleached pulp.

In the method for bleaching chemical paper pulp on a series of treating stages containing at least one stage for preparing a reaction mixture for treating pulp by using chlorine dioxide solution, a reaction mixture is kept under oxygen pressure. In this case, the bleaching is carried out under the following conditions: (1) oxygen pressure is kept in 3-10 bar, (2) chlorine dioxide is used in a ratio of 0.05-5 wt.% based on dried weight of pulp, (3) the temperature of the reaction mixture is kept to 40-90°C and (4) the viscosity of pulp is kept to 5-15%.