Mercury‐based ferrofluids are prepared by using sodium amalgam to reduce iron into mercury while agitating mechanically and magnetically by an ordinary magnetic stirrer. The field dependence of the room‐temperature magnetization is measured and used to determine the particle size distribution and its aging behavior. Samples prepared by the conventional method of electroplating into a mercury cathode are also studied for comparison. It is shown that the samples prepared by the two methods are very similar and the small amount of sodium remaining in the samples prepared from sodium amalgam can enhance the stability.