To investigate the corrosion behavior of Zr-1.0Nb-xSn (x=1.0, 1.5, 2.0 and 2.5wt.%) alloy system, the corrosion tests of Zr-1.0Nb-xSn alloys were carried out in steam at 400℃ for 125 days and in 70ppm LiOH solution at 360℃ for 180 days. The matrix microstructures of the test specimens were analyzed using TEM and the oxide structures on the test specimens were analyzed using XRD. It was found from the analyses that the more Sn content the alloy had, the faster it was correded and with the increase of Sn content in the alloy the fraction of t-ZrO2 to m-ZrO2 was decreased. It was also found that the alloys having more Sn showed more dislocation density than those having less.