The formation behavior of polycrystalline silicon thin films during the aluminum induced crystallization (AIC) was investigated by in-situ heating transmission electron microscopy (TEM). The electron dispersive X-ray spectroscopy (EDS) analysis of annealed sample was showed that layer exchange of the a-Si/Al film is occurred during the annealing. Furthermore, from the in-situ heating TEM observation and EDS analysis of as-deposited sample, it was confirmed the co-existence of Si and Al in a-Si/Al film and the lateral growth of crystalline Si grain. The mechanism of AIC and layer exchange were discussed from the experimental results and the phase diagram of Al-Si system.