In this paper,a novel method to detect single DNA molecule was proposed.A sandwich-type hybridization assay was performed on a streptavidin-coated substrate.A large number of biotinylated capture DNAs（c-DNAs） were first covalently bound to the substrate through the interaction between streptavidin and biotin.Then,the complementary target DNAs（t-DNAs） were hybridized with the c-DNAs attached to the substrate,followed by hybridization between t-DNAs and biotinylated first probe DNAs（p-DNAs） conjugated to streptavidin coated magnetic microparticles（MMPs）.In this case,one t-DNA binding one MMP could be observed with an optical microscope.The number of the MMPs is linearly proportional to concentration of t-DNA in the range of 5×10-16-1×10-14 mol/L.This size amplification method was simple and could be applied in other biomolecules.