This study was conducted for the development of marine fuels stability evaluation methods that most closely predict actual performance. The NOC spot test, which is based on the ASTM spot test and modified for wider application, was used in the investigation. Comparisons were made between the more widely known evaluation methods [Shell hot filtration test, Mobil centrifuge test, ASTM Method for Thermal Stability of U.S. Navy Special Fuel Oil, D 1661-64 (1977), and microscopic observation] and the NOC spot test, and fairly good correlations were found. In regard to performance tests, a purifier test and a filter test were carried out using laboratory apparatus that simulated actual field performance, and the spot rating of the NOC spot test correlated with both the amount of purifier sludge and the strainer clogging rate. The stability of fuels during tank storage was remarkably influenced by a scale effect in laboratory tank storage tests. V/S was the appropriate parameter for scaling the laboratory tests, where V was the volume of the fuel stored and S was the surface area of the fuel in contact with air.