The Al₂O₃-supported PtAg catalysts were prepared and evaluated for the dehydrogenation of n-butane at 550^°C in the presence of H₂. The PtAg/Al₂O₃ catalyst prepared by an impregnation method using the Cl- removing Pt/Al₂O₃ and AgNO₃ showed a higher activity and selectivity to butenes and 1,3-butadiene compared to the Pt/Al₂O₃ catalyst, but a large amount of coke (about 30 wt% versus the catalyst weight) was formed during the dehydrogenation. The free Ag metal on the prepared catalyst dramatically promoted the coke formation, because the dehydrogenation of 1-butene over the Ag/Al₂O₃ catalyst produced a large amount of coke. The Cl- addition to the Cl- free Pt/Al₂O₃ catalyst decreased the coke formation by the reaction of the free Ag particles and Cl to form AgCl which was inactive for the coke formation. The highest initial conversion (50.3%) was obtained with the selectivity to butenes and 1,3-butadiene (butenes = 80.2% and 1,3-butadiene = 5.9%) when the PtAg/Al₂O₃ catalyst modified with Cl- was used.