Molecular modeling at B3LYP/6-31G (d, p) level is utilized to studying the effect of some functional groups on the electronic properties of aromatic compounds such as benzene and heterocyclic aromatic compounds as pyridine and pyrimidine. Proposed compounds are conducted to study the effect of functional groups such as CHO, OH, Br, CH3 and NH2. Results are discussed in terms, the total dipole moment TDM and band gap energy ΔE are calculated for all structures. Also, electrostatic potentials ESPs are calculated for all model molecules as contour which obtained that there is a change in the negativity and hence in the reactivity of studied models. Furthermore, bond length and bond angle are also calculated. Functional groups show the ability to change the electronic properties of the studied organic structures.