To produce desired aggregate structures of copolymers, the copolymer is usually first dissolved in a common solvent that dissolves all the blocks. However, a solvent having the exact same solubility to all the blocks of a copolymer is rare. Hence, it is extremely important to know whether the block copolymer forms micelle in a common solvent, and if it does, to know the micelle's structure. In this study, we used polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) dissolved in dimethyl formamide (DMF) as a model system to address block copolymer micelle formation and its structure in a solvent good for all the blocks as DMF dissolves both PS and P4VP. Our atomic force microscopy (AFM) and cryogenic-transmission electron microscopy (cryo-TEM) results clearly demonstrated that PS-b-P4VP with a wide range of molecular weight and P4VP composition in DMF forms a spherical micelle. Furthermore, contact angle measurements and TEM results clearly show that the micelle has a PS core and a P4VP corona. In comparing the dry micelle and the micelle in DMF, we discovered that the micelle core is significantly swelled by DMF. Our findings suggest that soft-core micelles widely exist for block copolymers in solvents good for all the blocks but with significant selectivity between different blocks.