Conjugated polymers have excellent light-emitting properties that are useful for biological imaging and sensing applications. Here, we report a new way to form stable, water-soluble suspensions of conjugated polymers by encapsulating them in amphiphilic block copolymers. It was found that the folding property of conjugated polymers is a critical factor for solubilizing them into typical coil-coil block copolymer micelles. By introducing saturated bonds into conjugated polymers, they were readily encapsulated in block copolymer micelles, resulting in highly fluorescent polymer nanoparticles. The emission wavelength of fluorescent micelles can be tuned by controlling the number of encapsulated light-emitting polymers per micelle as well as changing the exciton delocalization length in conjugated polymers. This strategy has been extended to make multifunctional (i.e., fluorescent and magnetic) nanoparticles by encapsulating iron oxide nanoparticles in light-emitting polymers.