It is known that corrosion resistance of Zr-Nb binary alloy is sensitive to the microstructural changes with heat treatment. Therefore, microstructural observation and corrosion test were performed for the specimens that were heat treated with different cooling rate and annealing temperature. Corrosion resistance was deteriorated with increasing supersaturated Nb concentration in matrix and increasing area fraction of β_(Zr) phase. And it was observed that the amount of supersaturated Nb in matrix is more effective to the deterioration of the corrosion resistance than the amount of the β_(Zr) phase in the same alloy system. During the corrosion test in steam at 400℃ for 180days, the fine β_(Nb) phases were precipitated at α` needle boundary and twin boundary in the case of the specimen having martensitic structure, and the decomposition of β_(Zr) to (α_(Zr)+β_(Nb)) phase occurred in the specimen having Widmansta¨tten structure.