Temperature and melting conditions during flash fusing are studied. As a first step to clarify the microscopic behavior of toner, the temperature changes, considering thermal nonlinearity and the local melt viscosity dependence on the temperature changes, are examined. Consideration of the nonlinearity is performed by modification of the thermophysical properties employed in the temperature calculations, on the bases of the qualitative results of experiments. Suitable modification methods are a thermal conduction constant model and increment of the heat capacity model. The modification is necessary for the region from the start to the time the peak temperature appears on the toner surface. The local melt viscosity is analyzed by superposition of the calculated temperature changes and measured melt viscosity of the bulk toners. The melt viscosity varies a little with the kind of toner for the mid region in the toner layer. The melt viscosity calculated for the mid-region is one requirement determining the fixing strength. This means that to get good fixing strength, not only is it necessary to lower the melt viscosity, but also the thermophysical properties of the toner must be raised.