The electrostatic assembly of calf thymus DNA on multi-walled carbon nanotubes (MWNTs)-modified gold electrode via poly(diallyldimethylammonium chloride) (PDDA), a cationic polyelectrolyte, has been studied. Piezoelectric quartz crystal impedance (PQCI) technique was employed to monitor the assembly of DNA in real time. Electrochemical impedance spectroscopy was also used to characterize the modification process. It has been found that modification of the electrode with MWNTs has significantly enhanced the effective electrode surface area in addition to providing negatively charged groups for the electrostatic assembly of cationic polyelectrolyte. PDDA plays a key role in the attachment of DNA to MWNTs and acts as a bridge to connect DNA with MWNTs, though the direct adsorption of DNA on MWNTs has been observed. Moreover, the interaction between DNA and chlorpromazine hydrochloride was observed with PQCI technique. This shows that DNA assembled on MWNTs still has the ability to interact with drug molecules, which has great importance in the construction of new types of miniature DNA biosensors.