Monomethoxy poly(ethylene glycol)-block-poly(trimethylene carbonate) (mPEG3–PTMC11, Mn of mPEG=3.1×103 and Mn of PTMC=10.8×103g/mol) was synthesized by ring-opening polymerization of TMC using mPEG3 as an initiator and stannous octoate as a catalyst. The block copolymer has a broad melting range with a peak at 49.5°C and a heat of fusion of 47.6J/g. The heat of fusion normalized to the mPEG content is higher than that of the mPEG3 polymer, suggesting that PTMC segments are also semi-crystalline. The mPEG3–PTMC11 films were stable in water at room temperature, whereas at 37°C the film specimens disintegrated and the amphiphilic block copolymer self-assembled into micellar-like nanoparticles with average sizes up to 210nm. The critical association concentration (CAC) of the formed micellar-like particles is 1.35×10−3mg/ml. The average size and polydispersity index of the formed mPEG3–PTMC11 nanoparticles depend on temperature and storage time: the values decrease with increases in temperature and in storage time. By co-dissolving dexamethasone with mPEG3–PTMC11 during the film preparation, micellar-like nanoparticles loaded with dexamethasone can be obtained after the film to micellar-like nanoparticles transition. A high loading efficiency of 93.3wt.% was achieved. The sustained release of the drug was complete in 20d.