The fluorescent detection system has been introduced into the study on denaturation/reassociation process of DNA fragments in gel, for improving In-Gel Competitive Reassociation technique, one of genome subtraction methods. The annealing behaviour of the mixture of 3'-Fluorescein-labelled and 5'-Cy5-labelled DNA fragments was analysed by Fluorescence Resonance Energy Transfer (FRET) technique from donor Fluorescein to acceptor Cy5. We showed that two fluorescent dyes labelled at 3' and 5' ends of DNA fragments caused FRET in both the solution and the gel. The characterisation of fluorescence-labelled fragments in gel and the changes of their fluorescence intensity will be reported.