The mechanism of nodulation in copper electrorefining process was investigated by experimental method and simulation, in particular as for its growth in height. Due to the high current density at the tip, the nodule height increased as an exponential function of the time for electrolysis. Therefore, the growth behavior of nodules was strongly affected by the size of the nucleus and the existence of the threshold size to lead an electrical short circuit was suggested. Since some nodules obtained in the industrial process included mold releasing agent carried from anode-casting process, mold releasing agents are considered to be one of the main causes of the large nodule and need to be removed for the improvement in the current efficiency.