Numerical simulation method is proposed to predict fix strength on a destruction test of fixed image on a xerography fusing system. The 2-Dimensional ALE (Arbitrary Lagrangian-Eulerian) fluid and non-isothermal simulation can solve toners transitional deforming on a fuser nip and calculate a contact area between the toner and a paper, which takes into account involved factors: viscoelasticity of the toner and its temperature dependence, thermophysical properties of the toner and a fusing material, loading conditions, and fusing temperature etc. In addition, the structural simulation is applied to clarify how a fusing material fit the roughness on the paper structure and is combined with the fluid simulation. The combined method can calculate the contact area on images of both high and low density, which agrees with experiments of that when the type of toner is changed. Additionally, the results show that roughness on a paper structure affects the fix strength which strongly depends on the contact area. This method can be used to design fusing systems and toner properties.