The testing was carried out to obtain accurate values of thermal conductivities of insulation systems for use of a Liquefied Natural Gas (LNG) carrier. As preliminary examination, the small-sized specimens of component insulation materials were tested at various temperatures between the LNG temperature of about-160°C and the ambient temperatures of +20°C and +28°C. The test materials were chosen for the experiments on the uses of the rigid polyurethane foam, phenol foam and glass wool, and sizes of specimens were 160 mm in diameter and 15 to 30mm in thickness. After the preliminary examinations for the component single material described above, the thermal conductivities of selected systems were tested with the large-sized specimens subsequently with the intention of evaluating the influence of irregularities on thermal conductivity caused by joining the same materials or compounding the different materials with different thermal conductivity characteristics. The specimens had a section of the “as-built” insulation of 2. 5×2. 0m and 200 mm thickness. The testing was carried out with specimens in the horizontal position and in the full temperature difference condition keeping the upper end at about-160°C and the lower end at +20°C and + 28°C. Finally, the tests to confirm the values of the thermal conductivity of large-sized specimens were carried out with the small-sized specimens cut from the large-sized specimen. The paper presented deals with the both experiments described above, and the results obtained are summarized as follows ; 1) The thermal conductivities obtained from the experiments with large-sized specimens consisting of a section of the “as-built” insulation for ship of 2. 5×2. 0m and 200 mm thickness indicated the values of ; panel system 1 λ₁=0. 0184±0. 0014Kcal/m·hr·deg (+28/-161°C) panel system 2 λ₂=0. 0197±0. 0015Kcal/m·hr·deg (+20/-160°C) which were the average one for the total thickness of insulating materials with the thermal gradient from the LNG to ambient temperature. The difference in results reflects the influence on the over-all thermal conductivity from irregularities by joints and reinforced materials in systems. 2) The thermal conductivity obtained from the experiment with large-sized specimen (panel system 1) with full thermal gradient from-161°C to +28°C was well in agreement with the estimated value based on the test results with small-sized test-pieces cut from the large-sized specimen.