The invention relates to a method for treating a fibre stock for making of paper, board or the like. The method comprises obtaining a fibre thick stock and adding to the fibre thick stock at least one cationic first agent, and adding separately to the fibre stock and after the addition of the cationic first agent, at least one anionic second agent, which is an water-soluble anionic copolymer of acrylamide, methacrylamide or acrylonitrile, in such amount that the ratio of the added absolute cationic charge to the added absolute anionic charge is from 1:0.1 to 1:0.95. The invention relates also to the product prepared by using a fibre stock, which is treated by using the method.