In order to predict the deformation process of a liquid droplet impacting on a solid surface at room temperature, the mathematical model has been built up including the effects of surface tension as well as slip mechanism of contact line working in accordance with the degree of wettability on the solid/liquid interface. The numerical droplet deformation process has been compared with the experimental data for some Weber number cases and has been found to be in fairly good agreement not only in the spreading process, but also in the subsequent reducing process. It has been proved that the present model is enough to predict the droplet deformation process. Also, it has been clarified that the droplet deformation process on the solid surface is strongly influenced by the effects of the surface tension as well as the slip phenomenon to keep a proper contact angle.