This paper was aimed to prepare, characterize and determine the thermal energy storage properties of poly(methyl methacrylate) (PMMA)/n-nonadecane (C19) capsules as a novel micro/nanoencapsulated phase change material (M/N-EPCM). The M/N-EPCM was fabricated via emulsion polymerization reaction of methylmethacrylate (MMA) monomer occurred around C19 used as core material. The chemical structure of the prepared M/N-EPCM was verified using Fourier transform infrared (FTIR) spectroscopy method. The analysis results obtained from polarized optical microscopy (POM) and scanning electron microscopy (SEM) indicated that the synthesized micro/nanocapsules had virtually spherical shape. The particle size distribution (PSD) analysis indicated that the prepared M/N-PCM(1:2) with the highest nonadecane content had mean diameter of 8.18 mu m and 4.90 (v/v%) nanosized capsules. These properties make it a feasible PCM in incorporation to thermal fluids, textile, building and food packaging materials. The differential scanning calorimetry (DSC) analysis results showed that the micro-nanoencapsulated nonadecane had a suitable melting temperature and of 31.23 degrees C and a total latent heat value as high as 139.20 J/g, respectively. Additionally, the prepared M/N-EPCM had good thermal durability, thermal reliability, chemical stability, phase change reversibility properties even after extended 5000 thermal cycling, and also reasonably thermal conductivity. Therefore, it had a promising potential in different thermal energy storage application such as solar thermal controlling of building envelopes, thermal protecting of vehicle battery systems, and thermal regulating applications. (C) 2014 Elsevier Ltd. All rights reserved.