In this paper, the thermal characteristics of multilayer insulation (MLI) were experimentally investigated by using boil-off calorimetry method for seeking optimum standards of thermal insulation conditions. It is necessary to design the thermal insulating efficiency for applying to cryogenic instruments such as HTS power cable system. It is well known that the thermal characteristics and heat transfer of MLI are greatly affected by various MLI structures such as the number of layers and layer density, etc. However, it is difficult to know the thermal characteristics of MLI correctly. The heat leak by MLI between room temperature and liquid nitrogen temperature was measured at various conditions using a cylindrical cryostat. The cryostat consists of two guard vessels located at both end sides and a test vessel between them. The guard vessels are also filled with liquid nitrogen to prevent radiation heat leak through the both end side of the cylindrical test vessel to measure the heat leak only through MLI