Single grains of Si crystal are manipulated, for the first time, in solid‐state crystallization of amorphous Si films. The artificial nucleation sites are introduced by employing the self‐ion implantation. The nucleation and growth, during the subsequent thermal annealing strongly depends on the ion dose and energy. Preferential growth of the single nucleus takes place exclusively at each site and continues to propagate over them. Consequently, the location of grains is controlled, and the size distribution is remarkably shrunken in contrast to the films crystallized by random nucleation.