The　crystallization　mechanism　in　an　amorphous　Ge2Sb2Te5　(a-GST)　thin　film　induced　by　a　single　pulsed　laser　was　investigated　in　this　paper.　The　finite　element　simulation　and　x-ray　diffraction　analysis　showed　that　two　kinds　of　crystallization　mechanisms　performed　for　the　laser-induced　phase　transition　of　a-GST,　that　is,　the　solid-state　phase　transition　took　place　at　a　lower　laser　fluence　while　the　liquid-solid　phase　transition　occurred　at　a　relatively　high　laser　fluence.　Transmission　electron　microscopy　observations　showed　that　the　microstructure　in　the　liquid-solid　phase　transition　was　more　uniform　as　compared　to　that　in　the　solid-state　phase　transition　because　of　poor　atom　diffusion.　Crystallization　characteristics　at　different　laser　fluences　and　film　thickness　were　elucidated.　It　was　found　that　at　a　lower　laser　fluence　a　thinner　film　had　the　better　crystallinity　owing　to　thermal　convection,　while　at　a　higher　laser　fluence　a　thicker　film　showed　the　better　crystallinity　due　to　the　release　of　latent　heat　in　the　liquid-solid　phase　transition.　These　findings　enable　a　deep　understanding　of　ultra-fast　phase　transition　induced　by　laser　irradiation.　(C)　2018　Laser　Institute　of　America.