Aqueous dispersions of thermosensitive gold nanoparticles protected by diblock copolymers of poly(ethylene glycol) and polyphosphoester were prepared and studied. Diblock copolymers MPEG-b-P(EEP-co-PEP) with different compositions that are composed of monomethoxy poly(ethylene glycol), random copolymer of ethyl ethylene phosphate (EEP), and isopropyl ethylene phosphate (PEP) were synthesized by ring-opening polymerization in bulk. Thioctic acid seas then conjugated to the terminal hydroxyl group of the polyphosphoester block by esterification. Gold nanoparticles were then prepared by a one-step method and showed core-shell structure with an average gold core diameter of about 10 nm surrounded by a MPEG-b-P(EEP-co-PEP) shell with a thickness of about 30 rim. These polymer stabilized gold nanoparticles are reversibly thermosensitive in aqueous medium, exhibiting tunable collapse temperatures which are dependent on the composition of the diblock copolymers. Methyl tetrazolium (MTT) assay against HEK 293 cells demonstrated that these gold nanoparticles are with good biocompatibility. These gold nanoparticles protected by thermosensitive diblock copolymers with tunable collapse temperature are expected to be useful for biomedical applications.