For developing fusing system in electrophotography, estimation of fusing quality such as minimum fusing temperature and gloss becomes difficult in case that fusing conditions (pressure, temperature, dwell time) and toner property change. In this study, simple analytical model has developed based on toner deformation considering viscoelastic properties of temperature and frequency to investigate mechanism of fusing performance. Consequently, the index:ε, the quantity of toner viscoelastic deformation, well estimates the fusing qualities for various parameter set under the same paper roughness and same toner mass per area condition.