M OLECULAR modeling at B3LYP/6-31G (d, p) level is utilized to study 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 supposed to study the effect of some functional groups such as CHO, OH, Br, CH 3 and NH 2 . Total dipole moment (TDM) and band gap energy(∆E) are calculated for all supposed structures. Also, electrostatic potentials (ESP) are calculated for all model molecules as contour. From all results, 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 showed their ability to make a change in electronic properties of the studied structures.