The morphology of polymeric nanoparticles prepared by polymerization‐induced self‐assembly (PISA) is depended on the degree of polymerization of the solvophilic and solvophobic blocks. Herein, a series of poly (N,N‐dimethylaminoethyl methacrylate)‐b‐poly (benzyl methacrylate) (PDMA‐b‐PBzMA) diblock copolymer spherical nanoparticles were synthesized via reversible addition‐fragmentation chain transfer (RAFT) mediated PISA. These diblock copolymer nanoparticles are with nearly the same hydrodynamic size and solvophobic chain length, but with different solvophilic chain length. We used these nanoparticles to stabilize the oil‐in‐water Pickering emulsion. We find that the stability of Pickering emulsion increases with the length of solvophilic chain of the nanoparticles. Moreover, the droplet size of the Pickering emulsion can be tailored by varying the oil/water ratio and concentration of nanoparticles.