We investigated the effect of extensive compositional change on the structure and magnetic properties of the post-annealed FexPt100-x (48≤x≤68) thin films with and without MgO underlayer deposited at room temperature. It was found that, during annealing, the FePt films on MgO underlayer with Fe-rich composition around 59at% exhibit much faster ordering than those with other compositions although the single layer FePt films were found to have the optimum composition for the chemical ordering of only slightly Fe-rich composition of 52at% of Fe. The film of MgO(10 nm)/Fe59Pt41(8 nm) after annealing at 600℃ for 30 min showed perpendicular magnetization with coercivity about 800 kA/m and the squareness was close to 1 due to the formation of (001) oriented ordering phase. The results of FePt thin films on MgO underlayer deposited at room temperature present a striking contrast with those of FePt grown on heated MgO single crystal substrate, where the optimum composition for the ordering was around 62at% of Pt. An interpretation on the observed experimental phenomena is given in this paper.