Co-solvency and Co-nonsolvency are two extreme phenomena in the field of polymer solution. The former is the phenomenon that a mixture of two nonsolvent forms a good solvent for a polymer, and the latter is the reverse rare phenomenon that the solubility of polymer in a mixture of good solvents vanishes. The origin of co-nonsolvency has been debated for decades. The argument now is concentrating on whether or not the mixed solvent structure can influence polymer solubility. To clarify this, we propose to use poly(n,n¿-diethyl acrylamide) (PDEA) water/ethanol solution as a model system, and small angle neutron scattering (SANS) as the main tool to reveal the origin of co-nonsolvency. From the three Flory-Huggins interaction parameters in the model system, contribution of mixed solvent structure in the solubility of PDEA can be evaluated as a function of d-ethanol content and temperature.