A novel bi-functional fluorescent labelling agent derived from 5-hydroxy-1-naphthalene sulfonic acid and tris(2-amino-ethyl)amine was prepared and incorporated to 5'-monophosphate function of DNA through phosphoramidate bond. The labelling agent was designed to bear fluorescent moiety as well as primary and tertiary amines which are expected to have positive charge at neutral pH range. A 15-mer DNA labelled with this reagent exhibited enhanced duplex forming ability with its complement compared to the corresponding un-labelled DNA.