A variety of sub-10 nm nanoparticles are successfully prepared by crosslinking of polystyrene-b-poly(1,3-butadiene) (PS-b-PB) and polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) block copolymer micelles and inverse micelles. Among them, the core-crosslinked PS-b-PB micelles can self-assemble into ultrathin (< 10 nm) macroporous (pore size <1 mu m) membranes in a facile way, i.e., by simply drop-coating the particle solution onto a mica surface. No continuous/porous membranes are produced from shell-crosslinked PS-b-PB micelles and both forms of PS-b-P4VP micelles. This suggests that the unique structure of the block copolymer precursor, including the very flexible core-forming block and the glassy corona-forming block and the specific block length ratio, directly determines the formation of the macroporous membrane.