An improved method of modifying keratin fiber represented by wool is disclosed. Mechanical force is applied to keratin fiber in the presence of an aqueous solution of a transition metal salt to weaken and destroy the structure of the parts under the keratin layers in scales (surface cells) so that the transition metal may be introduced in the under-keratin layers locally at high a concentration. Then, the keratin fiber is immersed in a bath containing oxidizing agents such as hydrogen peroxide and monopersulfuric acid. The oxidizing agents are decomposed by catalytic effect of the transition metal. Mainly due to pressure of oxygen gas evolved by the decomposition keratin layers, which are scales on the keratin fiber, are peeled off. Thus, it is possible to remove only the keratin parts, which are not agreeable to the touch, without damaging the non-keratin protein, and to provide modified fiber which substantially contains no remaining metal. High shrink proof effect may be achieved without using the conventional chlorine compounds as the oxidizing agents.