The effect of thermal processing on corrosion behavior of N18 alloy has been investigated by autoclave tests in 0.01 mol/L LiOH aqueous solution at 360 ℃ and l8.6 MPa, and the microstructures of these specimens were examined by TEM and SEM. The results show that the corrosion resistance of specimens is improved obviously by $\beta$ phase quenching before cold rolling and annealing due to nano second phase particles (SPPs) precipitated dispersively, but when the intermediate annealing temperature increased to 740 ℃ before cold rolling and annealing, the corrosion resistance is lowered greatly due to SPPs coarsened to several hundred nanometers, where the corrosion rate accelerates markedly after the corrosion transition at 150 d exposure. The existence of β-Zr phase in the microstructures of specimens treated at 780 and 800 ℃ for 2 h in dual phase region is harmful to the corrosion resistance, but the corrosion resistance returns to a better level after the decomposition of β-Zr phase during the following processes of cold rolling and annealing at 580 ℃ to obtain fine SPPs.