We propose a discrete model for analyzing a temperature field during a flash fusing. The model is composed of each packed toner particle on a paper surface with assumed spherical shape and discrete coordinates. The contact condition for each particle and paper defines thermal resistance. The advantage of the model is that particle characteristics for the toner layer are considered, and more accurate analysis of the surface and interface toner temperature is possible compared with the previous layered model. First, the discrete particle model is introduced. Then, temperature changes of the toner layer during flash fusing are examined by the discrete particle model and important conditions for obtaining good fixed images are established. The calculated internal temperature in the toner layer shows that the temperatures at the top of the layer are greatly increased and similar for each particle group; however, the temperatures at the bottom of the layer vary significantly for the particle groups. The particle group structured from a smaller number of particles shows a higher bottom temperature. We suggest that to avoid smeared images and to obtain high print quality, the toner layer should be controlled to have small and uniform particle number.