In this paper several different working gas mixtures for GEM-TPC were evaluated based on a Garfield simulation. Among them, Ar:CH4:CF4=90:7:3 (named herein TU gas) was selected for a detailed study because of its better performance. Some performances of drift velocity, transverse diffusion, spatial resolution and the effective number of electrons in various electric fields were obtained. The performance of a GEM-TPC prototype working in the TU gas was studied and compared with that in Ar:CH4=90:10 (P10 gas).