Polycrystalline silicon obtained by the crystallization of thin amorphous silicon films has been an important material for microelectronics technology during the last decades. Many properties are improved in crystallized amorphous silicon compared to the as-deposited polysilicon such as larger grain size, smoother surface, and higher-carrier mobility. In this work, the crystallization of amorphous silicon is investigated by combining transmission electron microscopy (TEM) observations and molecular dynamics calculations. TEM observations on a series of specimens have shown that the majority of the silicon grains are oriented with a < 110 > zone axis normal to the surface. In order to understand the crystallization mechanism molecular dynamic simulations were performed. It is found that the < 110 > c/amorphous interface exhibits the lowest reduced interfacial energy density while the < 111 > c/amorphous has the lowest reduced energy differences per unit interfacial area. The most energetically unfavorable interface is < 001 > c/amorphous.