Abstract: After being treated in different ways, Zr-Sn-Nb zirconium alloy specimens were exposed in 70 μg/g Li⁺ LiOH aqueous solution at 350 ℃ and 16.8 MPa. The microstructures and second phase particles of these specimens were observed by transmission electron microscope (TEM). The specimens treated by 580 ℃/C.R/500 ℃ show best corrosion resistance among all the specimens. TEM observation shows that a large number of second phase particles exist in the specimen, which are fine and dis-persed homogeneously, result in the lowest Nb content in the matrix.