All-atom molecular dynamics simulations were performed on 4-heptyl-4'-cyanobiphenyl (7CB) to study the mechanism of heat conduction in this nematic liquid crystal atomistically. To describe 7CB properly, the AMBER-type force field was optimized for the dihedral parameter of biphenyl and the Lennard-Jones parameters. The molecular dynamics calculation using the optimized force field well reproduced the experimental values of the isotropic-nematic phase transition temperature, density, and anisotropy of the thermal conductivity. Furthermore, the contributions of convection, intramolecular interaction, and intermolecular interaction to the thermal conductivity were determined by performing thermal conductivity decomposition analysis. According to the analysis, the contributions of convection, bond stretching, and bond bending interactions were higher in the direction parallel to the nematic director than that perpendicular to the director, which is the origin of the anisotropy in the nematic phase. This result indicates that the anisotropy is caused by well-aligned covalent bonds and high mobility parallel to the director. This quantitative description of the mechanism of heat conduction of 7CB is foreseen to provide new insights toward designing highly thermally conductive liquid-crystalline materials.