Large DNA molecules are known to exhibit a conformational change between the coil and globule states with the addition of polyethylene glycol (PEG). We investigated the coil-globule transition of T4DNA by observing the response of the molecule to a well-defined elongational flow field, where the response was monitored by the flow-induced birefringence. The flow-induced birefringence (Delta n) increased with the strain rate for solutions of low PEG concentration, indicating that the DNA molecule is in an extended coil state. There were no an response at high PEG concentration solutions, where the DNA molecule is in a well-developed globule state. The dependence of Delta n on PEG concentration is in agreement with the PEG concentration dependence of DNA size, which was directly measured by a fluorescence micrograph method. A comparison of the results by the flow birefringence method with those by the fluorescence micrograph method indicated that the globule of the DNA molecule near the coil-globule transition is deformed by the elongational flow field. The elongational flow technique was proven to be a useful method for investigating the coil-globule transition of large DNA molecules.