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The syntheses, crystal and electronic structures, and linear optical properties of the new orthoborates LiMBO3 (M = Sr, Ba) are reported here. These compounds, which crystallize in the monoclinic space group P21/n with cell dimensions a = 6.476(2), b = 6.684(3), c = 6.843(3) Å, β = 109.41(3)°, and Z = 4 for M = Sr, a = 6.372(1), b = 7.022(3), c = 7.058(1) Å, β = 113.89(1)°, and Z = 4 for M = Ba, are modeled in terms of the cluster units (LiMBO3)2. The calculated electronic structures show that the top of the valence band consists of mostly the O 2p orbitals and the bottom of the conduction band consists of cationic orbitals. The dynamic refractive indices of these orthoborates are obtained in the framework of the INDO/SCI approximation together with the “sum-over-states” method. It is found that the refractive index is larger for LiSrBO3 than for LiBaBO3 and the charge transfer from O2- anionic orbitals to cationic orbitals appears to provide significant contribution to the linear polarizability of these compounds.