Fe-C coating is electroplated on the surface of a long period grating (LPG) optical fiber, and the growth of passive film on its surface is monitored through the change of transmitted spectrum. The test is performed in saturated calcium hydroxide solution for 50 h, during which the passivation of the Fe-C coating is monitored with electrochemical impedance spectroscopy, and the change of the transmission spectrum of Fe-C coated LPG fiber is recorded with an optical spectrum analyzer (OSA). Results indicate that an increase is observed in open-circuit potential (OPC), passive film resistance, and transmission intensity while a decrease is present in passive film capacitance and resonant wavelength, all of which are associated with the growth of passive film. This Fe-C coated LPG optical sensor can be used to monitor the growth of passive film on steel in aqueous environment with varying sensitivity from 0.04 to 0.12 nm thickness increase corresponding to 1.0 nm decrease in the resonant wavelength.