A polycrystalline silicon thin film with a high crystallinity was obtained using ferritin with a Ni core $\left(7\mathrm{nm}\right)$ , which enabled us to precisely control the density and position of the nucleus for crystal growth. The core density of ferritin adsorbed on the amorphous silicon surface was controlled in the range from ${10}^9{\mathrm{cm}}^{-2}\text{to}{10}^{11}{\mathrm{cm}}^{-2}$ . Crystal growth was performed at $550°\mathrm{C}$ in ${\mathrm{N}}_2$ . Crystallinity or grain size strongly depended on Ni core density. Polycrystalline silicon film with the average grain size of $3\mu \mathrm{m}$ and a high crystallinity was obtained at a low Ni atom density of ${10}^{12}{\mathrm{cm}}^{-2}$ .