We have studied the photonic band of dual_periodic one_dimensional(1D) photonic crystals. The band splits into many sub_bands when we modulate periodically the thickness of the unit of the 1D photonic crystal. Some narrow bands appear in th e gap of the simple photonic crystal with single_period. The field of the electr omagnetic modes in the split bands exhibits dual_periodicity. The modes in the n arrow bands are localized, but those in the wider bands are extended.