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A precise information on spatial distributions of temperature are important for diagnosing power transformers and evaluating its life because a power system failure will result in an enormous loss in tangible as well as social. The optical fiber Bragg grating (FBG) sensors, which have been studied intensively for last decade, can be very efficient tools for applications to above mentioned purpose because these are immune to EMI and can be highly multiplexed, which enables efficient quasi-distributed temperature sensing along tens of km range. We proposed a fiber-optic temperature monitoring system with an array of over 4 sensor gratings. The temperature-induced Bragg wavelength variations are accurately monitored by a scanned tunable wavelength filter. Differential measurement with temperature-stabilized reference gratings and a Gaussian curve-fitting algorithm has been used to enhance measurement accuracy, which obtained temperature resolution of ~0.6degC, and linearity error less than 0.4 %.