Computational treatment of the effects of air existing in a nip region is studied for accurate temperature estimation during toner fusing process in electrophotographic printers. Thermal resistance of the existing air in paper surface roughness and between toner particles is evaluated and thermally equivalent air layer with uniform thickness is introduced based on the amount of the air. Four models are proposed to discuss an adequate setting position of the air layer in a computational region. Toner surface temperatures estimated by the models are compared with measured ones during fusing process. It is clarified that the estimated temperature shows a good agreement with the measured ones with the following two models: 1) The air layer based on paper surface roughness is positioned at toner-roller interface and that based on the toner particles arrangement is in the midst of toner layer. 2) The air layer based on paper surface roughness is the same position with model 1) and the toner is treated as a porous uniformly including air. The above two models are applied to another two kinds of print media with different surface roughness, offering good estimations. The results confirm the usefulness of the treatment of existing air for temperature estimation during fusing process. Effects of toner particle sizes for color printing on the temperature field are also discussed with the estimating calculation. It is expected that the smaller size of toner particles offers an improvement of fuse quality for high-speed color printing with lower energy consumption.