We present a systematic study on the adsorption of NO and CO in Cu-ZSM-5, using an ab initio embedded cluster methodology at the B3LYP level of theory. We found that the effects of the cluster size and Madelung potential are small for adsorption energies of CO and NO. For adsorption of CO, the calculated binding energy of 32 kcal/mol is in good agreement with experimental data from 29 to 32 kcal/mol. On the contrary, for adsorption of NO the calculated binding energy of 22 kcal/mol is much smaller than the experimental estimate, though it is consistent with recent experimental observation that NO binding energy should be smaller than that of CO. Madelung potential, however, is important for obtaining the correct blue shift of an adsorbed CO and red shift of an adsorbed NO.