We present a facile and versatile approach to prepare diblock copolymer based Janus nanoparticles (NPs). Diblock copolymer PS-b-P4VP can self-assemble into patchy particles with P4VP protuberances on their surface using the emulsion solvent evaporation method. After cross-linking the P4VP protuberances, Janus NPs are produced by disassembling the particles. Within the P4VP domains, other species including metallic and inorganic are preferentially grown to extend composition and functionality. The Janus NPs are amphiphilic and capable to self-organize into superstructures. The size ratio of P4VP/PS and thus the Janus balance of the NPs are tunable by changing block chain length ratio of the block copolymers.