Flooding is one of the major disasters that occurred in Indonesia. According to Disaster Data National Disaster Management Agency (BNPB) in 2018, there were 871 floods in Indonesia causing losses that were not less material and non-material. Gajahwong river located in the Province of D.I. Yogyakarta experiences flooding almost every year, this greatly affects the safety and comfort of the people around the Gajahwong river. Seeing the facts in the field, flooding problems on the Gajahwong river are a serious problem that needs attention. The steps that can be taken are disaster mitigation. In disaster mitigation planning due to the Gajahwong river flooding, one effort is to make a prediction map of flood inundation areas. In this study prediction modeling of inundation and buildings affected by Gajahwong river floods using 2, 10 and 25 years return period discharge with ArcGIS Pro 2.4 software, ArcGIS Desktop 10.6.1 and HEC-RAS 5.0.1. The purpose of this study is to model the area prediction, model the affected buildings and the three-dimensional visualization of the Gajahwong River flood. The results obtained in this study at the 2, 10 and 25 years return period showed that in Q2 the inundation area was 0,5277 km2 and affected buildings were 1203 units. in Q10 the inundation area was 0,7358 km2 and affected buildings were 1578 units and in Q25 the inundation area was 0,8448 km2 and the affected buildings were 1770 units.