An extensive study of rare-earth perovskite BaPaO3 and BaNpO3 has been performed by first-principles tactics based on density functional theory (DFT), because the delocalized f-electrons play an important role in the band structure formation, to reveal their impact on the overall physical and chemical properties; it has turned out to be an interesting theme. Along with critical radii and thermoelectric properties, two different theories are employed to calculate the structural properties. The DFT and empirically calculated lattice constants are in rational accord with the experimental results. The critical radius calculations show that the BaPaO3 lattice has a smaller oxygen migration activation energy than the BaNpO3. In addition, we discuss the band profile and magnetic moments for these materials, which demonstrate the half-metallic ferromagnetism with a direct energy gap of 3.91 eV for BaPaO3 and an indirect gap of 3.79 eV for BaNpO3. More interestingly, the integral magnetic moments are in accordance with the Slater–Pauling rule.