Abstract: The aromatic strength of 5-element hereocyclic compounds an d pyridines were analysed qualitatively and quantitatively,and the sequence of a romative strength was given.The sites of 5-element heterocyclic compounds and 6 -element heterocyclic pyridine at which close electron substitutive reaction ta kes place were found and proved .The reason why close electron substitutive reactio n takes place more easily in one compound than in another was analysed.