Based on solidification analysis in which mechanical properties of quasi-solid alloy and compensation of liquid alloy in dendritic boundries are considered, solidification process of alloy is divided to four stages, which are quasi-liquid stage, compensated stage, uncompensated stage and dendritic bridging stage Hot tearing happened in uncompensated stage In order to investigate hot tearing process, SEMA was carried out for hot cracks in directionally solidified Al-Cu and Renel25 alloys Dendritic bridges were discovered in those cracks And tracks caused by dendritic bridges broken were also discovered in crack sections Therefore, hot tearing is proved to be caused by dendritic bridge breaking and developed through cavities between bridges joining together