Spectroscopic and viscosity methods were applied to investigate the interaction between methylene blue (MB)-Sm(III) complex and herring sperm DNA by using acridine orange as a spectral probe in Tris-HCl buffer (pH 7.40). By means of molar ratio method, the binding ratios between MB-Sm(III) and DNA were determined as nSm(III):nMB =1:3, nMB-Sm(III) complex:n DNA = 8:1, and the apparent molar absorption coefficient of εMB-Sm(III)-DNA was found to be 7.01×10⁵ M^-1•cm^-1. The bonding constants at different temperatures K^θ292K = 7.35×10⁵ M^-1, K^θ310K = 1.25×105 M^-1 were obtained by double reciprocal method. Thermodynamic function computation demonstrates that ΔrSm^Ө is the primary driving power of the interaction between MB-Sm(III) and herring sperm DNA. By combination analysis of the Scatchard method, circular dichroism spectroscopy and viscosity method, it suggests that the interaction mode between MB-Sm(III) complex and herring sperm DNA are a mixed binding, which contains partial intercalation and electrostatic interaction.DOI: http://dx.doi.org/10.4314/bcse.v26i3.8