A semi-empirical formula is proposed which may well describe the complex phenomena of the absorption and scattering of weak beta rays emitted by the radioactive sampies of different thicknesses mounted on planchets.The formula obtained as a result of a theoretical consideration of the back-scattering of the sample mount and the backscattering,the self-scattering as well as the self-absorption of the sample itself is I=m/(am+be^-cm) where Ⅰ is the observed counting rate in counts perminutem is the weight of the sample over a fixed area,and a,b and c are constants.