The thermal behavior of poly(lactic acid) (PLA) was studied by near-infrared imaging to provide a molecular-level understanding of the physical improvement caused by nanoclay dispersion. A set of PLA samples, each having different nanoclay dispersion, was prepared under varying sonication time. Crystallinity variation of the polymer interacting with the nanoclay particles was analyzed by a roundtrip temperature scan below the melting temperature. Namely, the samples underwent heating and then cooling in the opposite way during the spectral measurement. The discrepancy of the spectral feature between the heating and the cooling indicated the development of the hysteresis associated with the cold crystallization of the PLA lamellae. The generation of the spectral residuals revealed the inner working mechanism of how the polymer structure undergoes variation depending on the presence of the clay particles and their dispersions. The sonication brings substantial dispersion of the nanoclay over the polymer matrix. The nanoclay particles then induce the additional development of the crystalline structure due to the molecular interaction between the PLA and nanoclay arising from the presence of enormous surface area, which in turn induces variation of mechanical strength to the polymer.