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An optical method for the evaluation of temperature distribution of multilayer insulation (MLI) is presented. Four fiber arrays with four Fiber Bragg Grating (FBG) sensors in each array are fabricated in a wavelength division-multiplexing (WDM) scheme. MLI temperature varies the gratings periods of the FBG sensors which changes the back reflected Bragg wave length. This can be correlated with temperature change. Because of the small size of theses sensors with a diameter of around Ø 125 μm this kind of temperature measurement is very well qualified to investigate the temperature distribution in a multilayer blanket. For demonstration, a calorimeter with a test cylinder (Ø 219 mm · H 1820 mm) insulated with aluminized Mylar (20 layers) is used. The axial and transverse temperature distribution of MLI is recorded for various vacuum levels and compared with calculated temperature values. The results obtained were well agreed with the theoretical calculations.