Sub-macroscopic fractal structure can be found by scanning electron microscopy and transmission electron microscopy on most areas of the surface of a dislocation-free bicrystal silicon ribbon grown by a special process using a micro-twin as a seed. The size of a single fractal cluster is several micrometres to several tens of micrometres, with a fractal dimension, D_f, of about 1.64-1.84. The nature of the fractal structure changes with increasing thermodynamic driving force and gradually approaches that of snowflake-like figures. Preliminary theoretical analysis implies that the fractal structure should result from an inherent mechanism in bulk crystal growth and is not a surface phenomenon.