A fiber optic senor for carbon content measurement of the molten steel in a basic oxygen furnace (BOF) is described. The sensor includes a fiber optic cable for transmission of the optical radiation from the radiation collection head to the multi-wavelength detection unit located in remote area from a furnace. The radiation collection head includes a telephoto lens aimed at the furnace mouth to collect, and thus enhance, the effective radiant energy emitted from the hostile environment adjacent a furnace as well as to reduce spurious radiations from other nearby furnaces. The detection unit based on emission spectroscopy converts the radiations into digital signals and the signal information processed to determine the carbon content of the steel contained in the metallurgical furnace. The Preliminary results after online carbon content sensing are discussed.