Abstract: The reaction characteristic of catalytic oxidative dehydrogenation of ethane over a silica supported heteropoly acids is examined.It is observed that there exists a ignition temperature for ethane oxidative dehydrogenation.When the reaction is carried out over this temperature,the reaction is controlled by a free radical gas phase mechanism.The acidity of HPA controls the ethane adsorption on the surface.The stronger the acidity,the higher the ignition temperature of radical reaction.