A prototype Fiber Bragg Gratings (FBG) sensor for monitoring corrosion of steel structures was presented and the performances were characterized at different humidities. The sensor was fabricated by sputtering and electroplating a sensitive metal film on FBG side-face. The surface micro-structures of sensor at different humidities were investigated by Scanning Electron Microscope during a 27-day experimental cycle. The shifting of wavelength was recorded by Optical Spectrometer Analyzer in two environments with humidity of 60% and 99% RH. The wavelength of FBG sensor at 60%RH increased linearly with an increment of 0.025 nm per day. For sensors at 99%RH, the chirping appears, which implies more serious corrosion occurs than that at 60% RH. The experimental results show that the proposed FBG corrosion sensor is applicable for corrosion monitoring at a humidity of lower than 60%RH, and applicable for early alert of corrosion at a higher humidity.