Various test specimens were prepared by austempering low alloyed Mo-Ni-Cu ductile iron blocks of high graphite nodule count at 270, 320 or 370℃ for 0.5, 1, 3 or 9hrs. Tensile test, CVN impact test and plane-strain fracture toughness test(compact tension specimen of 50㎜ W) were done for each heat treatment condition at room temperature. X-ray diffractometer and optical microscope were used to investigate the change of microstructure and relationships between microstructure and test results. The highest retained austenite volume percent at each austempering temperature was corresponded to the highest mechanical property. The highest elongation value of 17%, U.T.S. value of 1,600 MPa or K_(IC) value of 90㎫ √m were reached at each optimum condition. The best heat treatment condition for fracture toughness were 3hrs` holding time combined with the austempering temperature of 270 and 320℃, and 1hr`s of 370℃.