To mimic nanostructures assembled by biomolecules in organic cells and achieve precise self-assembly of block copolymers, a simple but valid way is introduced to quasi-quantificationally control the aggregation numbers (N-agg) of polymeric micelles. A three-dimensional and closed microconfinement similar to a cell is constructed by W/O inverse emulsion as the spot for self-assembly of the pH-responsive block copolymer poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP). The N-agg values of the resulting polymeric micelles are effectively controlled by tuning the number of polymer chains encapsulated in isolated water pools. Micelles with different N-agg values are successfully prepared and characterized by atomic force microscopy, transmission electron microscopy, and dynamic light scattering. When the number of polymer chains enclosed in a water pool (N-chain) is less than the average N-agg of normal micelles generated in bulk aqueous solution, the resultant aggregates formed in the confined spaces always have lower N-agg as well as smaller sizes than the normal micelles do, while normal micelles predominantly form when N-chain > N-agg (normal micelle).