Recently the ductile cast iron is being used successfully to the parts for processing machinery vessels and gear etc. This study is mainly concerned with the heat treatment for the specimens of crank shaft which are made of ductile cast iron. The results obtained are summarized as follows. Comparing the mechanical properties of the specimens for the normalized ductile cast irons the specimen heat treated at $550^{circ}C$ was the best for crank shaft of air-compressor. After austenizing at $910^{circ}C$ it was observed that the higher the reheating temperature is the less tensile strength and the hardness became which was supposedly attributed to the fact that the amount of pearlite. Austenite matrix was reduced by reheating after normalizing and that as the reheating tem-perature went up the pearlite generated was less and the distance between the pearlites were widened at last made pearlite globular. In the comparsison of crank shaft for air compressor made of ductile cast iron with that made by forged steel the crank shaft made of ductile cast iron was superior in economical terms. And ductile cast iron could be practically enough if only the elonga-tion which was inferior mechanical property to forged steel could be reinforced by increasing the diameter of crank pin when designing the crank shaft.