An in-line production method for providing a liquid flow of at least one additive in the short circulation and into the liquid flow of a paper making stock of a fiber web machine by feeding, the liquid flow of the at least one additive to the liquid flow of the short circulation, wherein a suitable amount of a microfibrillated cellulose or nanofibrillated polysaccharide is provided substantially simultaneously with the feeding of liquid flow of the at least one additive.