The magnetic and microstructural properties of Ta/FePt/C thin films after annealing at various temperatures have been investigated. The results indicate that for a very thin Ta buffer with amorphous structure and relatively large roughness, defects and grain boundaries of high density are formed in the FePt film grown on it, which effectively lowers the energy barrier of the ordering with Ta interdiffusion into the grain boundaries of the FePt film during the annealing process. The formation of the L10 phase of FePt thin films is thus greatly promoted with the ordering temperature of FePt significantly reduced by introducing a thin Ta buffer layer. On the other hand, when Ta layer is relatively thick, its roughness accordingly reduces, and most of the interfacial Ta atoms are finally bounded with the advent of the crystalline structure of the Ta buffer. Therefore, Ta buffer layer at this thickness range has little effect on the ordering of FePt films.