A rapid method to measure the distribution of mass density of paper and paper board was developed. Paper is irradiated with electron beams with energies from 50 to 200 keV and the attenuation of the electron beams by the paper is detected by a photographic film and a real-time TV monitor system. The image of the attenuation of the electron beams (electrograph) is converted into the distribution of mass density through a basis weight calibration. By this method, the basis weight distribution of paper and paper board from 0 to 400 g/m2 was obtained in less than 1 second with both detectors. It was found that the electrograph gives very similar information as beta-radiograph. Experiments to optimize the imaging of the distribution of mass density of paper by electron beams are described along with the relevant physics.