nJ(Se,Se) (n=1–4) nuclear couplings between Se atoms were analyzed by using molecular orbital (MO) theory as the first step to investigating the nature of bonded and nonbonded nJ(Se,Se) interactions between Se atoms. The values were calculated by employing Slater-type triple ξ basis sets at the DFT level, which were applied to structures optimized with the Gaussian 03 program. The contribution from each occupied MO (ψi) and ψi→ψa (ψa=unoccupied MO) transition was evaluated separately. 1J(Se,Se) was calculated for the MeSeSeMe model compound, which showed a typical dependence on the torsion angle (ϕ(CMeSeSeCMe)). This dependence explains the small values (≤64 Hz) of 1Jobsd(Se,Se) observed for RSeSeR′ and large values (330–380 Hz) of 1Jobsd(Se,Se) observed for 4-substituted naphtho[1,8-c,d]-1,2-diselenoles, which correspond to synperiplanar diselenides. The HOMO→LUMO and HOMO−1→LUMO transitions contribute the most to 1J(Se,Se) at ϕ=0 and 180° to give large values of 1J(Se,Se), whereas various transitions contribute and cancel each other out at ϕ=90° to give small values of 1J(Se,Se). Large 4Jobsd(Se,Se) values were also observed in the nonbonded Se⋅⋅⋅Se, Se⋅⋅⋅SeO, and OSe⋅⋅⋅SeO interactions at naphthalene 1,8-positions. The Fermi contact (FC) term contributes significantly to 4J(Se,Se), whereas the paramagnetic spin-orbit (PSO) term contributes significantly to 1J(Se,Se). 2J(Se,Se) and 3J(Se,Se) were analyzed in a similar manner and a torsional angular dependence was confirmed for 3J(Se,Se). Depending on the structure, the main contribution to nJ(Se,Se) (n=2, 3) is from the FC term, with a lesser contribution from the PSO term. Analysis of each transition enabled us to identify and clearly visualize the origin and mechanism of the couplings.