Despite great progress in the non-classical homogeneous nucleation and crystallization theory, the heterogeneous processes of atomic nucleation and crystallization remain poorly understood. Abundant theories and experiments have demonstrated the detailed dynamics of homogeneous nucleation; however, intensive dynamic investigations on heterogeneous nucleation are still rare. In this work, in situ transmission electron microscopy (TEM) at the atomic scale was carried out with temporal resolution for heterogeneous nucleation and crystallization. The results show a reversible amorphous to crystal phase transformation that is manipulated by the size threshold effect. Moreover, the two growth pathways of Bi particles can be mainly assigned to the atomic adsorption expansion in the amorphous state and effective fusion in the crystal contact process. These interesting findings, based on a real dynamic imaging system, strongly enrich and improve our understanding of the dynamic mechanisms in the non-classical heterogeneous nucleation and crystallization theory, providing insights into designing innovative materials with controlled microstructures and desired physicochemical properties.