We study the ligand effect on the LUMO/HOMO of ligated zinc porphyrin (ZnP) by utilizing DFT. The ligated ZnPs are zinc meso-tetraphenylporhyrin (ZnTPP), zinc tetrabenzoporphyrin (ZnTBP), zinc meso-tetraphenyltetrabenzoporphyrin (ZnTPTBP) and five-coordination ZnTPP with imidazole (ZnTPP(Im)). The ligands decrease LUMO-HOMO energy gap (ΔE_g) of ligated ZnPs with respect to non-ligated ZnP, which is in good agreement with experiments. LUMO of ZnTPP and ZnTBP is stabilized by LUMO+1 (b₁-type, C_2v-symmetry) of respected ligands, while their HOMO is destabilized by HOMO−4 (a₁, C_2v) and HOMO (a₂, C_2v) of respected ligands. ΔE_g of ZnTPP and ZnTBP can be adjusted by changing the ligand's electronegativity, such as halogen substitution at the ligands. Halogenated ZnTPP has ΔE_g smaller than ZnTPP, which confirms the experiments. Geometry change in ZnP, which allows the intermolecular perturbation in the frontiers molecular orbitals, shifts LUMO/HOMO energies as it is shown for ZnTPP(Im) and ZnTPTBP. The calculated ΔE_g of ZnTPP(Im) and ZnTPTBP are greater than that of ZnTPP and ZnTBP.