The high strength of aluminum alloy can be achieved by applying heat treatment aging. Nevertheless, the exact effect of the most common defect, bifilm, to the response of aging has not been known yet. Therefore, a research to know the effect of bifilm to the response of its aging on the most used aluminium alloy, ADC12, is conducted. In this research, ADC12 specimen casting was done three times with different stirring variations before liquid aluminium was poured to get various amount of bifilm. There were three variations: without stirring, three minutes stirring, and five minutes stirring. Ingot was used as the specimen comparison. These four specimens were given various treatments: T6 artificial aging (aging to form hardening precipitates), T7 artificial aging (over-aging), and without aging. The responses made by the alloy contained of bifilm were observed by using optical microscope and Scanning Electron Microscope (SEM). Element mapping observation of α-Aluminium phase that was far and near the bifilm was done. Microhardness Vickers testing was done on α-Aluminium matrix on areas both far and near the bifilm to obtain additional information regarding microhardness that correlates to precipitates from artificial aging. The T6 artificial Aging will increase tensile strength and hardness of the ADC12 aluminum alloy by transforming θ (Al2Cu) and Q (Al5Cu2Mg8Si6) phases into θ' (Al2Cu) and β' (Mg2Si) precipitates, which is coherent with its matrix, α-Aluminium. The result of the elemental mapping shows that there are different precipitate distributions of θ' and β' after the treatment of artificial aging on specimens. The α-Aluminium matrix microhardness from T6 artificial aging on specimens that contain almost zero bifilm is 101 HV, while on specimens that contain approximately 100 mm is 67 HV and 63 HV on areas near and far from bifilm. The T7 artificial aging treatment also shows the same microhardness pattern, which is on specimen that contains almost zero bifilm is 65 HV, and on specimen that contains approximately 100 mm is 55 HV (on both far and near the bifilm). It implies that bifilm affects the response of the artificial aging.