The formation of an inclusion complex of methylene blue with b-cyclodextrin is testi®ed by voltammetry and spectroscopyand its stable constant is determined to be 4.4×103M－1. The interactions of methylene blue and the inclusion complex withDNA have been investigated by means of voltammetry. The binding constants of methylene blue and the inclusion complex toDNA are obtained through the changes of peak currents and their values are 1.1×104M-1 and 7.2×103M－1, respectively.The experimental results reveal that the inclusion complex does not decompose when it interacts with DNA, and bcyclodextrinitself does not interact with DNA.Their binding numbers (n) are 1 both methylene blue and the inclusioncomplex. According to the experimental data, it can be inferred that the binding model of methylene blue to DNA may be`electrostatic binding' under our experimental conditions. This can clearly explain the experimental phenomena.