Currently, definition of LOI(Loss on Ignition) is “weight rate of incinerable parts among residues after incineration” as stipulated in Wastes Control Act. LOI means weight rate of unburned matter among residues after incineration, and as the rate of LOI is lower, it is judged as operation ability of incineration facility is good. In addition, LOI is an important factor to decide capacity of incineration facility. However, as currently performed test method of LOI has controversial issue which may lead to confusion to tester for mathematical formula to calculate the result value, I would like to suggest improvement through LOI test on residues occurred from domestic incineration facility. In this research, I analyzed residues from 3 units of 2 facilities of daily wastes and 5 units of 5 facilities of industrial wastes, and compared current test criteria and the criteria to be improved. Current test criteria of LOI was revised in 2015. And to understand difference of 2 criteria, I analyzed test criteria before/after revision simultaneously and understood validity. In addition, to understand difference between LOI formula and organic content formula based on current LOI test criteria, I calculated LOI and organic content based on test result. From the result of LOI analysis of residues, difference between 2 criteria was 1.23% in average. This difference is not a big gap, but it is judged that the test method after revision which performs LOI and pollination experiment simultaneously is more valid for the part of probability compared to test before revision which performs LOI and pollination experiment separately. And the calculation result of LOI and organic content is same, so it is judged that the formula should be simplified to reduce confusion. keyword: Loss on ignition, Incineration residues, Thermal treatment