The excess surface free energy per unit area or interfacial tension is determined for hard-rod isotropic and nematic phases coexisting at equilibrium. The number density and order parameter profiles across the interface are obtained by minimizing the free energy in the inhomogeneous fluid using an approach that generalizes Onsager＇s classical theory to spatially inhomogeneous hard rod solutions. The excess surface free energy is a minimum when the director in the bulk of the nematic phase is at a 90 degrees angle to the surface normal, indicating that this tangent alignment is the preferred orientation likely to be-observed in practice. The surface tension for a tangent director is found to be 0.316k(B)T/(dL), where hg is Boltzmann＇s constant, T the absolute temperature and d and L are the diameter and length of the rods. The dependence of the rod concentration on position across the interface is nonmonotonic.