The invention concerns a method and apparatus for real-time measurement of refiner mechanical pulp quality index such as freeness. According to the method, refiner mechanical pulp transported in a steam-phase carrier is illuminated with electromagnetic radiation, and the change in the radiation spectrum caused by refiner mechanical pulp is detected. According to the method, the change in the radiation spectrum caused by refiner mechanical pulp is measured at a minimum number of four wavelength bands and the measured intensities are computationally processed to obtain the desired index of quality.