The magnetic and electronic properties of Ba 2 MnMoO 6 are investigated by using the full-potential linearized augmented plane-wave method, with both generalized gradient approximations (GGA) and GGA+U approaches. The calculated results indicate that parameter U plays a vital role in the electronic structure characterization. The ground state is found to be antiferromagnetic and insulating. In terms of the Heisenberg model, the nearest-neighbor and the next-nearest-neighbor magnetic interaction parameters are determined to be J 1 =−1.17meV and J 2 =−0.87meV. The valence states of Mn and Mo are suggested to be divalent and hexavalent respectively, in agreement with the experimental data.