The invention relates to a method of treating chemical pulp fibers by a polymerizing hydroxy acid, the fibers thus obtained and the products refined from them. In the method, the hydroxy acid reacts with the reactive groups of the fibers in the presence of a catalyst, forming ester bonds. The following units of the same hydroxy acid are oligomerized and/or polymerized to these grafted acid residues. The fibers thus treated and the products refined thereof endure better processing stages that include drawing and stretching than untreated fibers.