Selective oxidation of ethane by oxygen was examined over silica catalysts supporting cesium and bismuth. Aldehydes of C1-C3 were obtained with total selectivity of about 40% at ethane conversion lower than 5%. The catalytic performance was strongly dependent upon the bismuth loading on the catalysts. The higher turn-over-rate and the higher selectivity to aldehydes were observed at the Bi loadings lower than 0.5%. Characterization of the catalysts showed that isolated or highly dispersed Bi on the catalyst surface was indispensable for the high aldehyde yield. Cesium also plays an important role for the high ethane reactivity and the high aldehyde selectivity. A reaction pathway was proposed in which ethane is oxidized into acetaldehyde, but not through ethylene, and acrolein is formed through a cross-condensation of acetaldehyde and formaldehyde.