We have calculated the electronic structures of FePt and Fe0.9375Mn0.0625Pt using first principle calculations based on density functional theory within the local-spin-density approximation (LSDA). The Curie temperature ( $T_{c})$ was calculated by a simple model based on the mean field approximation. A supercell structure, containing eight unit cells, was used to calculate the density of states. The saturation magnetization ( $M_{s})$ and magnetocrystalline anisotropy constant ( $K$ ) at 0 K, and $T_{c}$ for Fe0.9375Mn0.0625Pt were calculated to be 942 emu/cm3, $8.24 \times 10^{6}$ J/ $\text{m}^{3}$ , and 590 K, respectively. The temperature-dependent $M(T)$ and $K(T)$ of Fe0.9375Mn0.0625Pt were calculated using the Brillouin function and Callen–Callen experimental relation, respectively, to obtain an $M_{s}$ and a $K$ of 822 emu/cm3 and $6.29 \times 10^{6}$ J/ $\text{m}^{3}$ at 300 K, respectively. These values meet the required $M_{s}$ and $K$ of 800 emu/cm3 and $5.00 \times 10^{6}$ J/ $\text{m}^{3}$ for achieving 4 Tb/in2 heat-assisted magnetic recording media (HAMR).