The characteristic curves of effective refractive index of infinite and finite photonic crystal are calculated and analyzed by using the dispersive relation and transmission coefficients respectively. The results show the dispersive curve of one-dimensional photonic crystals have the normal dispersion in the transmitted bands and abnormal dispersion in the band gaps which is similar with that of elastic electronic dipolar. Based upon our calculations, the explanation of some contradictive places or the difference in the relative literatures is given out.