In this study, we design different inscribed/circumscribed circular torus-type microchannels to investigate the stretching behavior of DNA molecules. Strain rate and relaxation time play an important role in DNA stretching. In order to perform an analysis of the coil-stretch transition of DNA, we develop a method of stretching DNA molecules by using £gPIV and CLSM measurements. £gPIV is designed to measure the velocity distribution, after which the local strain rate can be estimated. The hydrodynamic stretching of DNA molecules in the elongation flow is observed using a confocal laser scanning microscope (CLSM). The relaxation time of the DNA molecules is then estimated according to the CLSM images analysis. At present, our experiments using the electro-osmotic flow (EOF) driven at various electric fields and viscosities to stretch DNA molecules show how one can investigate the influence of hydrodynamic interactions in the case of stretching of DNA molecules.