Substituent Effect on Local Aromaticity has been studied in series of mono-and di-substituted biheterocycles (quinoline, isoquinoline, quinoxaline, quinazoline, 1H-Indole, 1H-Indazole, 1H- Benzo[d]imidazole). Electronically based indices (PDI, ATI and FLU) have been employed to investigate the substituent effect on the ?-electron delocalization in both heterocycle and benzenoid rings. Three typical substituents (Cl, OCH3 and CN) with different inductive and resonance effect have been selected. Generally, substituent causes a reduction in aromaticity irrespective of whether it is electron attracting or electron donating. Moreover, it is found that the substituent situation with respect to the heteroatom has a significant influence on the aromaticity, and irrespective of whether the two substituents form a meta or para isomer, they preferably choose the position which leads to the maximum aromaticity character. Also, influence of increasing the number of N as heteroatom and their sequences on ?-electron delocalization of aza derivatives of naphthalene and indole have been investigated by use of different quantitative descriptors including HOMA,NICS and electronic indices.