Polymethyl methacrylate (PMMA) encapsulated eicosanoic-stearic acid (EA-SA) eutectic nanocapsules were prepared by ultraviolet photoinitiated emulsion polymerization and various characterization techniques were employed to investigate the influence of preparation methods on thermal properties and particle size distribution (PSD). The results show that the particle size decreased and PSD narrowed with the increase of agitation speed, reduction of initiator and monomer concentration, existence of cross-linking agent and stabilized at agitated speed higher than 5000 rpm and monomer concentration lower than 0.15 mol/L. However, latent heats of the capsules decreased with the increase of monomer and initiator concentration. Type of emulsifier in emulsion has significant effects on PSD and phase change properties of EA-SA/PMMA and nonionic emulsifier is suitable for reducing particle size and enhancing heat storage ability. Morphology and chemical characteristic analysis indicate that spherical nanocapsules with average diameter of 46 nm were successfully fabricated and its maximum encapsulation ratio is 68.8 wt% without leakage of core material. The melting and crystallizing temperatures and latent heats of capsules were determined as 56.9 degrees C and 54.5 degrees C, 126.4 J/g and 128.3 J/g, respectively. Accelerated thermal cycling test shows that the nanocapsules have good thermal and chemical reliability after repeated thermal cycling. Besides, the super-cooling problem of PCMs reduced dramatically by forming nanocapsules. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.