The way of application is an important issue in thermal energy storage (TES) materials. Phase change materials are encapsulated to prevent leakage and to get application advantage. At the last decade, microencapsulated phase change materials (MEPCMs) are produced with functional outer surfaces. In this work preparation, characterization, and thermophysical property determination of microencapsulated n-octadecane in poly (styrene-co-divinylbenzene-co-acrylamide) shell was studied. N-octadecane core P(St-co-DVB-co-AAm) shell microcapsules were prepared with 0.5, 1, and 2% AAm contend to make the outer surface functional. Divinylbenzene was used as the cross-linker to produce stability and unimodal microcapsule size distribution. Microcapsules were synthesized by miniemulsion polymerization method, and characterized using Fourier transform infrared spectroscopy (FT-IR), polarized optical microscopy (POM), particle sizer distribution (PSD) instrument, differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA) techniques. The average melting and freezing onset temperatures of the MEPCMs were found slightly lower than the phase transition temperatures of n-octadecane core material as the enthalpy values were affected from the amide pendant group contend. Besides, produced microcapsules were found stable up to 148 degrees C.