Very good dependences of the variation in pi-electron delocalisation expressed by the aromatic character of the ring, defined by the geometry-based HOMA model and NICS index in mono 6-substituted fulvene and 8-substituted heptafulvene derivatives on the nature of substituent allows us for defining the substituent through resonance effect (by means of sigma(p)(+) and sigma(p)(-) scale of the substituent effect). The analysis is based on 35 optimised systems at DFT B3LYP/6-311 + G** level of theory.