Method for changing floc size distribution of a filler dispersion with a pre-treatment agent in producing paper, board or the like, comprising - obtaining an aqueous mineral filler dispersion, comprising particles of at least one first mineral filler, the filler dispersion having a floc size distribution with an original mean chord length value, - bringing a cationic pre-treatment agent, which initiates flocculation of the filler particles in the dispersion, into a contact with the particles of the first mineral filler, and forming a pre-treated filler dispersion comprising flocs, which comprise first mineral particles, - combining the pre-treated filler dispersion with a stock of fibres, characterised in that - the pre-treatment agent is a water-soluble synthetic polymer having an average molecular weight < 2 000 000 g/mol and selected from a group consisting of cationic polyacrylamide, glyoxalated polyacrylamide (GPAM), polyethyleneimine, polyamine, polyvinylamine (PVAM), polydiallyldimethylammonium chloride (poly-DADMAC), DADMAC-acrylamide copolymer, polyamidoamine epihalohydrin and any of their mixtures, - the charge density of the pre-treatment agent is 0.1 - 7 meq/g, determined at pH 7, whereby - the pre-treatment agent is used as one single liquid solution comprising the water-soluble synthetic cationic polymer, and optionally other second or following polymers, and - the pre-treatment agent is added to the aqueous mineral filler dispersion in an amount of 0.01 - 0.5 weight-%, calculated from the dry solids weight of the filler dispersion, whereby the floc size distribution of the filler dispersion is changed with the pre-treatment agent, so that the mean chord length value increases with 5 - 40 % from the original mean chord length value. - obtaining an aqueous mineral filler dispersion, comprising particles of at least one first mineral filler, the filler dispersion having a floc size distribution with an original mean chord length value, - bringing a cationic pre-treatment agent, which initiates flocculation of the filler particles in the dispersion, into a contact with the particles of the first mineral filler, and forming a pre-treated filler dispersion comprising flocs, which comprise first mineral particles, - combining the pre-treated filler dispersion with a stock of fibres, - the pre-treatment agent is a water-soluble synthetic polymer having an average molecular weight < 2 000 000 g/mol and selected from a group consisting of cationic polyacrylamide, glyoxalated polyacrylamide (GPAM), polyethyleneimine, polyamine, polyvinylamine (PVAM), polydiallyldimethylammonium chloride (poly-DADMAC), DADMAC-acrylamide copolymer, polyamidoamine epihalohydrin and any of their mixtures, - the charge density of the pre-treatment agent is 0.1 - 7 meq/g, determined at pH 7, - the pre-treatment agent is used as one single liquid solution comprising the water-soluble synthetic cationic polymer, and optionally other second or following polymers, and - the pre-treatment agent is added to the aqueous mineral filler dispersion in an amount of 0.01 - 0.5 weight-%, calculated from the dry solids weight of the filler dispersion, whereby the floc size distribution of the filler dispersion is changed with the pre-treatment agent, so that the mean chord length value increases with 5 - 40 % from the original mean chord length value.