Elastin-like polypeptides (ELPs) belong to a class of stimuli responsive polymers. Changes in temperature cause ELPs in aqueous solutions to undergo a reversible hydrophilic-to-hydrophobic transition. Previous experiments have shown that the transition temperature strongly correlates to the ionic strength of the solution. However, there have been few experimental reports on how the ELP molecules change their conformation as a result of changing the ionic strength of the environment. We here present our results on directly monitoring the conformational change of ELPs upon changing the ionic strength using a dual polarization interferometry technique. The linear ELP molecules were covalently immobilized onto a glass surface of a sensor chip via reactions of their primary amine group located at the N-terminal end, leaving the other end of the chain free to move in the solution. The DPI technique used two polarizations of light to measure the refractive index and the thickness of the ELP layer near the solid-liquid interface by measuring the interference between light waves. From this information, the conformational change of the ELP molecules was derived. It was observed that the thickness of the ELP layer initially had a slight increase and then dramatically decreased upon increasing the ionic strength of the aqueous solution, which could be explained by a conformational change of the ELP molecule from an elongated hydrophilic molecule to a coiled hydrophobic one. This observed phenomenon will be elucidated from a thermodynamic point of view.