Abstract: Suspected inclusion defect was found in TC11 titanium alloy forging during milling process. The forming cause for the inclusion was analyzed by microstructure observation and micro-area compositions analysis, and prefabricated inclusion smelting test was carried out to verify the forming cause. The results show that the suspected inclusion in TC11 titanium alloy forging was the embedded cutting tool fragment during the milling process,and the cutting tool fragment was formed at the tool tip due to the large moment feeding amount.