Many dam failure incidents that occur in the world are caused by seepage and piping. The research aims to find the effectiveness of the width of the dam core using asphalt concrete material. Researchers use the Geostudio SEEP/W program which will analyze seepage through the dam body by considering acceptance criteria so that the effective width of the dam core can be found using asphalt material. This research will test the width of the asphalt core between 50 and 100 cm at 10 cm intervals with several conditions of Minimum, Normal, and Flood Water Level. The results of this research are that the largest seepage discharge value is 1.50 × 10–6 m3/s and the seepage index value is 0.009. The width of the dam core that is effective for seepage safety conditions in the Dam body is 50 cm with the maximum permeability coefficient value being 1 × 10–8 cm/s. The results of the modeling that has been carried out show that the seepage discharge values with several dam asphalt core width values are mostly within the acceptance criteria. So, it needs to be investigated from other aspects such as resistance to flexibility and deformation.