We report a novel class of amphiphilic conjugated block copolymers composed of poly(3-octylthiophene) and poly(ethylene oxide) (POT-b-PEO) that exhibit highly tunable photoluminescence colors spanning from blue to red. POT-b-PEO self-assembles into various well-defined core/shell-type nanostructures as a result of its amphiphilicity. The self-assembly structure can be readily controlled by altering the solvent composition or by other external stimuli. The color change was completely reversible, demonstrating that the strategy can be used to manipulate the light-emission properties of conjugated polymers in a highly controllable manner without having to synthesize entirely new sets of molecules.