A precise evaluation of the insulation efficiency of a complex insulation system is one of the important problems in designing low-temperature gas carriers such as LNG, LE (Ethylene) 6, and LPG carriers. This paper deals with some experimental studies carried out for the purpose using heat flux meters designed and manufactured by one of the authors, and the results obtained are as follows : (1) The insulation test carried out on an large-size cut-out model having the same insulation system as a 1, 000 m³-capacity experimental LNG carrier proved a good agreement between measurements and calculations of both local and over-all K -values. (2) The cool-down test carried out as to the experimental LNG carrier and the experimental measurements carried out as to a 55, 000 DWT LPG carrier during her over a month service navigation not only confirmed the insulation efficiency, but also revealed the actual thermal conditions around the vessels and brought a lot of basic data available for the design use.