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There is a potential demand for optical current instrument transformer (OCT) in power industry. In the past two decades, R&D on OCT was focused on Faraday effect based technology. As a sensing technology, Fiber Bragg grating (FBG) was more and more attractive recently. For OCT, FBG and giant magnetostrictive material (GMM) based technology will be another choice. In this paper, this kind of power frequency AC current sensing system was established. One FBG was clung on a GMM rod that was exposed to the AC current induced magnetic field. The shift of FBG reflecting wavelength was interrogated by a wavelength to amplitude simultaneous interrogation system. Experiments with and without DC bias current have been done. The relation of the sensing current and the response of the system were defined by the transfer function of the system that revealed hysteresis loop character.