In the fusing process in electrophotographic machines, the temperature field and melting in the toner during fusing are important factors. As a first step leading to quantitative analysis, methods to measure the following conditions are developed: heat flux and unsteady temperature change on the toner surface by flash fusing, thermophysical properties of the toner and the paper, and toner deposit thickness and coverage. The measurement results are applied to the boundary conditions and thermophysical properties in an analysis using the finite element method. A model configuration is constructed with a toner layer and a paper layer. In this way, a one-dimensional unsteady temperature distribution is analysed. As a result of the analysis, the following points are made. During the time when fixing is performed, the maximum difference between the temperature on the toner surface and on the toner-paper interface is more than 200°C, regardless of the kinds of toner; this is attributed to very low thermal properties of the toner powder. At the same magnitude of fixing strength, the toner surface temperature level varies about 100°C with the kinds of toner. However, the toner-paper interface temperature is roughly the same regardless of the kinds of toner. Therefore, the interface temperature level is considered to be the lowest limit for fixing. On the other hand, the melt viscosity superposed on the temperature field varies little with the kinds of toner on the mid-region in the toner layer. It is also considered to be a requirement for fixing.