We develop a viable strategy to create uniform ultrastable perovskite/metal oxide core/shell nanoparticles with good control over the dimension by capitalizing on amphiphilic star-like triblock copolymers as templates. The diameter of the perovskite core and the thickness of the metal oxide shell can be independently and precisely regulated by tailoring the molecular weights (i.e., the lengths) of the inner and intermediate blocks of star-like triblock copolymers, respectively. The successful synthesis of perovskite/metal oxide core/shell nanoparticles is confirmed by a suite of characterizations. Moreover, metal/metal oxide core/shell nanoparticles are also synthesized using amphiphilic star-like triblock copolymer templates. The resulting perovskite/metal oxide nanoparticles exhibit the remarkable compositional, thermal, moisture stabilities. These perovskite/metal oxide and metal/metal oxide nanoparticles are then used in perovskite solar cells, with which improved device performance is observed.