The structure, electronic band structure, density of state, projected wave function, and optical properties of mullite-type orthorhombic Bi2M4O9 (M = Ga3+) crystals have been studied by applying density functional theory based on the Vanderbilt ultrasoft pseudopotential in the frame of the generalized gradient approximation as an exchange-correlation function. Satisfactory agreement between experimental and theoretical results indicates that the used method and conditions are suitable. M O bonds in tetrahedral MO4 environments are stronger and more covalent with respect to octahedral MO6; also Bi-O bonds in both studied structures are almost ionic in nature. The photocatalytic activity of Bi2Al4O9 and Bi2Ga4O9 is enhanced due to unequal values of Mulliken charges on the O atoms in MO4, MO6, and BiO6E groups. Bi2Al4O9 and Bi2Ga4O9 are direct and indifect band gap semiconductors with band gaps of 2.71 and 2.86 eV, respectively. Higher photo catalytic activity of Bi2Al4O9 is inferable from the lower effective masses of photogenerated carriers around the conduction band minimum and valence band maximum, in with Bi2Ga4O9. The presence of M and O orbitals in the valence and conduction bands reveals that symmetry breaking in the MO4 and MO6 units has an important role in separating charges and increasing photocatalytic activity. Photocatalytic activities of Bi2Al4O9 and Bi2Ga4O9 for decomposition of drganic pollutants and generation of hydrogen from water splitting are confirmed potentials. from band edge potentials.