The electronic structures of La 1 − x Ba x MnO 3 are calculated by using the all-electron full-potential LAPW method in the local spin-density approximation. The Jahn-Teller distortion is very important to give rise to the A-type antiferromagnetic ground state and the energy band gap for LaMnO 3. The ferromagnetic state is stabilized for La 1 − x Ba x MnO 3 without the Jahn-Teller distortion.