Abstract: The reason of hard spots were analyzed in die casting transmission case of ADC10 aluminum alloy by means of macroscopic and microscopic examination, chemical composition analysis, hardness testing, energy spectrum analysis and so on. The results showed that the hard spots were due to the oxidizing material or metallic compounds containing titanium, together with liquid aluminum injected in mould cavity and finally reserved inside of the section in the process of die-casting. After that, in the machining operation, the existence of hard spots caused damage of cutter, which caused cutting hardly to work normally, It even may cause seriously damage to scrap transmission case. To improve the pre treatment process of liquid aluminum, avoiding oxidizing material or metallic compound entered mould cavity, this would be effective to prevent the generation of hard spots.