The relationships between the microstructure changes, retained austenite volume and elevated temperature tensile properties of Mo-Ni-Cu ADI corresponding to various austempering temperatures and time were investigated, After the 250℃ tensile test for the test piece austempered at 270℃ the accicular bainite structure was observed blunted under room temperature microscope. In the case of 370℃ austempering, the feathery bainite lath spacing was observed broadened. But after the 450℃ tensile test, bainitic features could not be observed. As the testing temperature increased, retained austenite volume tested at room temperature decreased. Especially, after the 450℃ tensile test retained austenite volume approached nearly to zero. A little higher tensile properties appeared at 250℃ testing than those at room temperature.