An intramolecular charge transfer fluorescence probe of 4Ј-N,N-dimethylamino-4-amino-chalcone(DMAC) exhibits characteristics clearly correlated with the polarity of solvents. The interaction of this fluorescence probe with calf thymus DNA has been investigated. Generally, DMAC bound to DNA shows marked changes in fluorescence and absorbance properties compared to the spectral characteristics of the free form in solution phase. In the presence of DNA the fluorescence intensity of DMAC is greatly increased with a large bathochromic shift of excitation and emission wavelengths. A hypochromism in absorption spectrum was also observed. The absorption and fluorescence spectra, salt concentration effect, and KI quenching experiments demonstrate that DMAC molecule as an intercalator is inserted into the base-stacking domain of DNA double helix, and the interaction of the nucleobases with DMAC molecule causes the increase of fluorescence intensity and hypochromism in absorption spectrum. The intrinsic binding constant and the binding site number were estimated to be 7.04 ϫ 10 6 mol L Ϫ1 in base pairs and 0.065, respectively. The I/I 0 vs DNA concentration plot shows a linear range covering 1.98 ϫ 10 Ϫ6 to 2.08 ϫ 10 Ϫ4 mol L Ϫ1 in base pairs which can be used for determining DNA with a detection limit of 6.0 ϫ 10 Ϫ7 mol L Ϫ1 in base pairs (0.6 g ml Ϫ1 ).