It has been suggested that the phase transition of the poly(N-isopropylacrylamide) (PNiPAM) aqueous solution consists of at least two different processes: (1) a rearrangement of bound water around either hydrophobic or hydrophobic and hydrophilic group of PNiPAM and (2) a hydrophobic association of the hydrophobic groups in the PNiPAM chains or a residual interaction between the PNiPAM side chain residues. However, a single endothermic peak does not provide direct evidence for this suggestion. This study shows that the phase transition in PNiPAM aqueous solution comprises at least two processes by using temperature modulated differential scanning calorimetry. The single endothermic peak is separated into an endothermic and exothermic peak, and the heats from these peaks depend on the applied frequency. Curve fitting to experimental data interprets the transition mechanism of the PNiPAM aqueous solution in more detail, and the results and the limitation of the curve fitting are discussed.