The effect of various end-functional groups on the phase behavior of polystyrene-block-poly(n-butyl methacrylate) copolymer (PS-b-PnBMA) was investigated by using small-angle X-ray scattering, conventional and 3-D transmission electron microscopy, rheology, and Fourier-transformed infrared spectroscopy. A PS-b-PnBMA with maleic anhydride (SBM66-MAH) exhibited an unexpected thermoreversible transition occurring at higher temperature than its disorder-to-order transition temperature. The transition was very fast and induced a large increase in shear modulus. However, the transition at higher temperature was not observed in other PS-b-PnBMAs with carboxylic acid, diester, and hydrogen end groups.