L10 phase FePt alloy is regarded as one of the most promising materials for ultra high density magnetic recording media. However, structural point defects, which would reduce the media's signal to noise ratio, are inevitable in non-stoichiometric L10 FePt alloy. Hence, possible types of point defect (vacancy and anti-site defect) in non-stoichiometric ordered FePt alloy were fully studied using density functional theory. Investigation over vacancy shows the formation energy of Fe and Pt vacancy is respectively 2.58eV and 3.20eV. Geometry relaxation implies Fe vacancy has a stronger deformation force upon the original lattice. Meanwhile, anti-site defect study shows that the formation energy of Fe anti-site (Fe occupation in Pt sublattice) and Pt anti-site (Pt occupation in Fe sublattice) is respectively 1.05eV and 0.66eV. Therefore, for Fe-rich and Pt-rich alloy, the preferred structural point defects are both anti-site substitution rather than vacancy due to the much lower formation energy.