In this paper, polystyrene(PS)/n-dotriacontane(Dot) composite nanoencapsulated phase change material (NEPCM) for thermal energy storage was synthesized by a facile and time-saving ultrasonically initiated miniemulsion polymerization method. The influences of ultrasonic factors including ultrasonic power output, time and temperature, the ratio of composite emulsifiers composed by sodium dodecyl sulfate (SDS) and poly-(ethylene glycol) monooctylphenyl ether (OP-10), as well as the Dot/styrene ratio on the z-average particle size, conversion, encapsulation efficiency and the phase change enthalpy of NEPCM were systematically investigated. The morphology, chemical structure and thermal performance of NEPCM were characterized by particle size analyzer, transmission electron microscope (TEM), Fourier transform infrared (FT-IR), X-ray diffractometer (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TG), respectively. The results showed that the ultrasonic technique can be successfully applied for the synthesis of PS/Dot NEPCM with high conversion (95.02%) and excellent encapsulation efficiency (61.23%) in a relatively short reaction time (35min). The synthesized composite nanocapsules with uniform spherical core/shell structure and z-average particle size of 168.2nm, had a high latent heat capacity (Tm 70.9°C, 174.8J/g) and good thermal stability, which indicates that it has great potential for the thermal energy storage.