The authors determined the isothermal transformation diagrams in temperature ranges of 180-600°for structural Ni-Cr-Mo steel (0.52C, 1.58Ni, 0.71Cr, 0.56%Mo) and Cr-Mo steel (0.47C, 0.93Cr, 0.46%Mo) by micrography and hardness test, and then inspected the relation between isothermal transformation and retained austenite at room temperature, using the measurement of thermal dilatation during austempering as well as tempering after austempering. The results obtained are summarized as follows: (1) In the intermediate range (330-500°), austenite transforms to lower-bainite, wich finally breaks down to α+θ aggregate. (2) In the martensite range (below 300°), austenite firstly transforms to martensite during an surely period of cooling below Ms temperature, and then to lower-bainite isothermally, the latter transformation being delayed with the lowering of the hot-bath temperature. (3) After austempering at a fixed temperature in martensite range, the amount of retained austenite at room temperature increases with the time of holding in hot-bath, reaches a maximum at an intermediate period of lower-bainite formation, and then decreases.