Electronic structure and magnetic properties of Post-perovskite CaRhO3 have been studied by density functional theory calculations through using the projected augmented wave method. The Generalized-gradient approximation (GGA) calculations indicate a half-metallic ferromagnetic ground state and a low-spin configuration of Rh4+ ion with a magnetic moment of 0.57μB. While the experimental insulating antiferromagnetic ground state can be obtained only by GGA plus on-site Coulomb interaction U(GGA+U) calculations, which indicates the crucial role of corelation effect of 4d electron for the Post-perovskite CaRhO3.