Abstract: The nano-encapsulated phase change materials (NEPCMs) with polystyrene as shell and n-octadecane as core were synthesized by ultrasonic technique and miniemulsion in-situ polymerization.The influence of various polymerization factors on the morphology and thermal properties of nanocapsules was investigated systematically.The optimal reaction condition was 0.5% AIBN initiator (mass percent of oil phase,the same in the following), 0.4% DDT chain transfer agent, 1∶1 n-octadecane /styrene, and 2% total amount of composite surfactants consisting of 1∶1 SDS and OP-10.The results showed that the Z average diameter of capsules was 124 nm and its phase change latent heat was 124.4 kJ·kg-1.