In a method for producing nanofibrillar cellulose, cellulose based fiber material in dispersion is processed for separating fibrils. The method includes a first step where the fiber material is supplied to a disperser, where it flows through several counter-rotating rotors in such a way that the material is repeatedly subjected to shear and impact forces by the effect of the different counter-rotating rotors, and a second step, where the fiber material obtained from the first step is supplied to a homogenizer, where it is subjected to homogenization by the effect of pressure.