The impact of this research is to present results obtained from synthesis of nanoencapsulated Paraffin wax core/Thermoplastic elastomer Polyurethane shell with various particle size distribution via mini-emulsion polymerization that could be utilized in thermal energy storage. To achieve this objective, five different types of Polyurethane shell/Paraffin wax core nanocapsules have been synthesized based on Polyethylene glycol and Isophorone diisocyanate with Polycaprolactone assistance to improve fusion enthalpy and thermal performance. Process conditions including ultrasonic time and reaction temperature have been assigned differently to study effect of reaction parameters on PU/PW particle size distribution and time-temperature performance. Findings have shown that, increasing sonication time from 15 to 20 min and reaction temperature from 60 to 80 degrees C, mini-emulsion core droplet size has been decreased and narrow size distribution has been attained. Furthermore, molecular and morphology analysis and phase change thermal investigations confirmed polyurethane shell formation around spherical paraffin wax core. Moreover, thermal cycling test has been indicated high thermal performance and improved resistance of prepared core/shell, even after 100 heating/cooling cycles and approved its great potential for thermal energy storage applications. To conclude, narrow size distributed paraffin wax nanocapsules demonstrated effective and superior time-temperature history in contrast with broad size distributed nanocapsule. (C) 2020 Elsevier Ltd. All rights reserved.