Hydrogen bonded supra molecular structure is the essential feature of cinnamic acid (phenyl acrylic acid) while forming a layered crystal structure in solid state. It is an aromatic compound with strong hydrogen bonding between the carboxylic groups. The dimers are interconnected by CH….O intermolecular hydrogen bonds, with C-H unit mostly originating from aromatic ring. These bonds keep the dimers and higher oligomers together and thus the compound exists in crystalline form. Since the hydrogen bonded structure is susceptible to rupture at temperature changes above the ambient, a temperature dependent positron annihilation spectroscopic study has been performed to investigate the changes of its molecular structure. These subtle changes were investigated independently through differential scanning calorimetry (DSC), which sheds light on the activation energy of the molecular state at different temperature regimes and indicates the phase transition temperature.