Abstract：Elemental potassium was introduced to modify the MnO_x/γ-Al_2O_3 catalysts by impregnating method. The catalytic activities were studied in a continuous flow fixed-bed reactor under atmospheric pressure. The results indicated that the selectivity of benzaldehyde was improved significantly after the catalysts were modified. The optimum potassium loading on the catalysts is about 5% (wt). By means of XRD, XPS, NH_3-TPD, BET and IR, the effects of potassium on the catalysts of MnO_x/γ-Al_2O_3 were characterized. The XRD results reveal that the incorporated potassium acts on the Al_2O_3 to form new crystal K_2Al_2O_4 and that the valence state of manganese is not changed after the catalyst is modified by potassium, which is also verified by the XPS results. NH_3-TPD and FT-IR results show that certain amount of the potassium on the catalysts reduces the intensity and amounts of the acidic centers. The modified catalysts effectively inhibit the formation of toluene and benzyl methyl ether.