The purpose of this study is to clarify the influence of Mo and austenitizing temperature to the heat treatment aracteristics, using five (5) kinds of low alloyed ductile cast irons containing 0-2mass% of Mo. The specimens were austenitized at 1223K-900s and 1323K-900s, and the range of cooling time from Ac₃ to 773K was approximately 3-4000s. Each transformation regions of ferrite, pearlite, bainite and martensite existed in CCT diagrams for ductile cast iron use. And almost the same tendency at the austenitizing temperature of 1223K and 1323K was recognized. The ferritic and pearlitic transformation region in the CCT diagram shifted to a longer cooling time side with increasing Mo content because of the impurity solute drag effect of Mo. The bainitic transformation region existed even at a longer cooling time side in CCT diagrams in case of adding Mo. Ms temperature of low alloyed ductile cast iron was not influenced by Mo. With increase in austenitizing temperature, the ferritic transformation region in the CCT diagram shifted a little bit to the longer cooling time side, while the perlitic transformation region shifted to shorter cooling time side. It is considerable that it caused by increasing the amount of C in the austenite with rising austenitizing temperature. The bainitic transformation region was not influenced by austenitizing temperature. Ms temperature shifted slightly to the lower temperature by increasing the amount of C in the austenite with rising austenitizing temperature. For the both austenitizing temperature of 1223K and 1323K, the quenching hardness of 0Mo cast iron was 600 ～ 630HV10, and that of 2Mo cast iron was 700HV10. It is thought that the difference between these hardness is based on the increase in the amount of Mo carbide. When Mo was separately added to the low alloyed ductile cast iron, the influence of Mo on hardenability was little, and its influence of austenitizing temperature was also little.