Disclosed herein is a process for removing sulfur dioxide (SO.sub.2) and nitrogen oxides (NO.sub.x) from flue gas using chlorine dioxide (ClO.sub.2). In the process, chlorine dioxide is allowed to react with flue gas in an alkaline solution so as to be able to simultaneously remove sulfur dioxide and nitrogen oxides from the flue gas. In the simultaneous removal of sulfur dioxide and nitrogen oxides from flue gas using the process, chlorine dioxide is produced by side reactions, and the produced chlorine dioxide can be used to remove sulfur dioxide and nitrogen oxides from flue gas.