Phase transition kinetics and microstructures of AgInSbTe (AIST) and AIST- Si O2 nanocomposite applied to phase-change memories (PCMs) are investigated. In situ electrical property measurement found that the incorporation of Si O2 escalates the recrystallization temperature (Tx) of nanocomposite. Both X-ray diffraction and transmission electron microscopy showed grain refinement in the nanocomposite which, in turn, results in an increase of the activation energy (Ea) of phase transition, as indicated by subsequent Kissinger's analysis. Increase of T x and Ea in the nanocomposite was ascribed to AIST grain refinement and hindrance to grain growth due to dispersed Si O2 particles in the sample matrix. Johnson-Mehl-Avrami analysis revealed the decrease of Avrami exponent in nanocomposite, implying that the dispersed Si O2 particles promote the heterogeneous phase transition. Static I-V characteristics and reversible binary switching behavior of PCM devices not only confirmed the results of microstructure characterizations but also illustrated the feasibility of the AIST and its nanocomposite layer for PCM fabrication.