Modified Ga-α-Al₂O₃ catalyst with Ba showed high catalytic activity, selectivity and low carbon formation on catalytic ethane dehydrogenation to ethylene even in the presence of steam. The resultant Ba-Ga-α-Al₂O₃ (Ba/Ga molar ratio = 0.10) catalyst showed high ethylene selectivity (98%), high activity, and stability. Temperature-programmed oxidation measurements revealed that Ba addition suppressed coke formation at Ga sites. XRD, XAFS and TEM results showed the existence of highly dispersed β-Ga₂O₃ on the α-Al₂O₃ support, irrespective of Ba addition. Observation of hydrogen adsorption using FT-IR spectroscopy revealed a decrease in the surface tetrahedrally coordinated Ga (designated as Ga(T)) sites by Ba modification, indicating that the surface Ga(T) sites are covered with Ba. Density functional theory calculation revealed that coke formation through ethylene decomposition is likely to occur at surface Ga(T) sites, and revealed that the addition of an optimal amount of Ba to Ga-α-Al₂O₃ inhibits coke formation at the surface Ga(T) sites, leading to high ethylene selectivity.