The effects of steam pressure and preoxidation on the nodular oxidation of Zircaloy -4 were investigated. The accelerated oxidation of nodular type occured at 500℃ above the threshold steam pressure and the oxidation rate increased with increasing the steam pressure. When the uniform preoxide layer, which had been formed at 500℃ near 1 bar, exceeded the size of intermetallic precipitate, the nodular oxidation was drastically reduced at 500℃ under 100 bar pressure. The preoxide thickness protecting the nodular oxidation increased as the steam pressure of preoxidation increased. It revealed that the nodular oxidation may be related to the hydrogen build-up at the sites of the intermetallic precipitates within the uniform oxide surface. The build-up of hydrogen at these sites may lead to stress generation, resulting the cracking of the protective oxide. The localized rapid oxidation with the surface cracking may have caused the nodular corrosion