Blistering occurs when oxide scale swells during oxidation at high temperatures. Blistered scale causes surface defects when rolled. The present study investigated the effect of nitrogen on blister growth when steel is oxidized at high temperatures, and drew the following conclusions. Atmospheric conditions before oxidation affect blister growth. Blisters nucleate but do not grow, when a steel sample is held in Ar gas or in vacuum before oxidation. Blisters inflate when a steel sample is held in N₂ gas before oxidation. The gas inside the grown blisters is mainly N₂ gas. The steel surface is nitrided in N₂ gas at high temperatures. It is deduced that the steel surface is nitrided before oxidation, and the nitrogen component causes blister growth upon its release as N₂ gas at the scale/steel interface.