The object of the present invention is a method and arrangement in making of mechanical pulp to reduce the amount of organic dissolved and colloidal substances in the pulp making process water by treating a part of the process water. According to the method the treating is carried out by leading the process water filtrate, separated from the pulp by a press located before bleaching, to pre-treatment, where elongated fibres are fractionated from the process water to be treated. The filtrate that has passed the pre-treatment is led to membrane filtration, where at least part of the organic dissolved and colloidal substances included in the process water are separated from the rest of the process water. The membrane filtration concentrate i.e. the colloidal and dissolved substances separated from the process water, is led to further treatment and the permeate i.e. the process water that has passed the membrane filtration is led back to the pulp making process. An arrangement according to the present invention comprises means to carry out the method according to the invention.