The effects of nonmagnetic impurities (Kondo holes) in Kondo insulators (KI's) are studied based on the U=\ensuremath{\infty} Anderson model in the framework of a slave-boson mean-field theory under the coherent-potential approximation (CPA). The density of states for f electrons and its variation with the concentration of Kondo holes are calculated self-consistently. The specific-heat coefficient, residual resistivity, and the T=0 static susceptibility in the alloying system are obtained. The results show that the insulating gap in KI's can be easily smeared out by Kondo holes, and the system changes gradually from an insulator to a heavy-fermion metal with increasing concentration, which is in qualitative agreement with experiments. Furthermore, the appearance of an impurity band in the gap comes out naturally in the dilute limit of our CPA calculations.