The deflection radius is essential in determining residual stress estimations in flexible electronics. However, the literature provides only indirect methods for obtaining a deflection radius. In this study, we present a measurement methodology for directly measuring the deflection radius of a polyethylene terephthalate (PET) substrate (a popular substrate of flexible electronics) by using an optical interferometer. A Twyman-Green optical interferometer was established and phase-shifting technology was used to increase the measurement resolution. Five PET substrates with known deflection radii were prepared to verify the measurement precision of the proposed measurement methodology. The results revealed that the error variance of our proposed measurement methodology is smaller than 3.5%.