Aggregation behavior of poly(N,N-diethylacrylamide) (PDEA) in dilute aqueous solution has been studied by using the static and dynamic light scattering. It has been demonstrated that PDEA is not molecularly dissolved in water but forms molecular aggregates below the cloud point in one phase region. The association number of aggregates is of the order of 100, and the aggregate is suggested to be like a swollen micro-gel. The aggregation does not come from difficulty of dissolving a frozen structure in solid state, but the aggregates are also formed in solution starting from aggregation-free state by changing the solvent quality. The detailed aggregate size and structure depend on the dissolution process of the solid sample in making the solution. Once the aggregate is formed, it is stable, and does not change its association number and dimension for a long time. But, the association number may change with temperature and depends on thermal history.