Ab initio studies of structural and electronic properties of the monoclinic perovskite-type BiMnO3 and BiMn0.5Nb0.5O3 crystals are presented using the pseudo-potential plane wave method within the density functional theory in generalized - gradient approximation. The calculated equilibrium lattice parameters remain in a good agreement with the available experimental data. The gap energy took values in the range E-g = 0.2 - 0.5 eV for both compounds.