The aim of this study is to investigate the in situ encapsulation of n-hexadecane as a phase change material (PCM) with a polymethyl methacrylate (PMMA) shell through miniemulsion polymerization. n-Hexadecane (HD) and methyl methacrylate (MMA) form dispersed minidroplets in the aqueous phase, whereas PMMA is immiscible with n-hexadecane and separates through polymerization to produce hollow polymer particles filled with n-hexadecane. Different HD:MMA ratios and ethylene glycol dimethacrylate (EGDMA) as a cross-linking agent were considered to optimize the encapsulation process. A new approach by using novel equations from DSC, DLS, and microscopy analyses were developed to estimate shell thickness and the extent of contribution of MMA in the formation of the nanocapsules, pure polymeric particles, and also the void fraction of nanocapsules. It was found that the weight fractions of pure polymeric particles for the prepared samples were 12-17 wt %, and the shell thickness of the nanocapsules was about 12 nm. Also, the void fractions for these samples were 69-77%.