Silicon thin films of superior resistivity useful for, e.g., semiconductor elements in solar cells, are formed by coating and drying silicon vaporized in the presence of hydrogen alone or hydrogen and an inert gas, followed by contacting the thus produced silicon particles with solvent to form a silicon colloid. Preferably, the silicon colloid is produced by a process which comprises vaporizing silicon in an atmosphere consisting essentially of hydrogen and up to 10 mol of an inert gas per mol of hydrogen; condensing silicon vaporized in the first step to form fine silicon particles; bringing the silicon particles into contact with a solvent to cover the silicon particles with solvent, and collecting the solvent covered silicon particles to obtain the silicon colloid.