In this paper, we will study the partial regularity theorem for stationary harmonic maps from a Riemannian manifold into a Lorentzian manifold. For a weakly stationary harmonic map (u, v) from a smooth bounded open domain Ω ⊂ R m to a Lorentzian manifold with Dirichlet boundary condition, we prove that it is smooth outside a closed set whose (m − 2)-dimension Hausdorff measure is zero. Moreover, if the target manifold N does not admit any harmonic sphere S l , l = 2, ..., m − 1, we will show (u, v) is smooth.