In this work, we proposed an effective route, i.e., three-dimensional (3D) confined co-assembly of block copolymers (BCPs) and inorganic nanoparticles (NPs) within organic emulsion droplets, to efficiently encapsulate high-density and large-size NPs into the wall of BCP vesicles. Attributed to the interfacial instability, the aqueous phase could spontaneously enter into the organic emulsion droplets, thus inducing the formation of BCP vesicles encapsulated with NPs. Moreover, it is found that the encapsulation of NPs into the vesicle wall is mainly dominated by the ratio (DN/dw0) of the NP diameter (DN) to the layer thickness of the affinitive blocks (dw0). Utilizing the 3D confined co-assembly of polystyrene-block-poly

(4-vinylpyridine) and PS-coated gold nanoparticles (AuNPs) with different diameters, the high-efficiency encapsulation of AuNPs in the BCP vesicle wall at a DN/dw0 value of 0.66 can be achieved, which is greater than the previous reported critical value (i.e., DN/dw0 ≈ 0.5). Although the conformational entropy loss of the polymer chains increases with the introduction of AuNPs, the NPs can still be successfully encapsulated into the vesicle wall because the oil–water interface can suppress the diffusion of NPs to the aqueous phase. However, as the DN/dw0 value is further increased to 1.04, AuNPs will be repelled out of the vesicles because of the extremely strong entropic repulsion arising from the drastic loss in conformational entropy of PS blocks, thus forming some spherical hybrid micelles independently. Our results provide a new and facile route to realize the efficient encapsulation of large size NPs into polymeric vesicles.