The ionization energy and degree of ionization for Si and Ge with boron doping are calculated. The hole mobilities are then calculated as a function of doping concentration using the relaxation time approximation. When the screening effect is taken into account, the reduction of ionization energy results in the increase of degree of ionization. As a result, the calculated Si mobility becomes closer to the experimental data, whereas the calculated Ge mobility is almost independent of the screening effect. The inclusion of the broadening effect in the mobility calculation overestimates the ionized impurity scattering. As compared with the experiment, the screening effect is not avoidable to calculate Si and Ge mobilities, and the broadening effect must accompany with the hopping process.