Lima Puluh Kota Regency in West Sumatera Province is one of regencies in Indonesia that often has flood problem every year since last decade. In case of such large-scale flooding, it is important to classify the hazard zone for efficiency of the flood mitigation. In this paper, rainfall-runoff inundation (RRI) model is applied to the Batang Sinamar River Basin in order to predict the widespread inundation, where both rainfall-runoff from surrounding mountain and rainfall on flood-plain contributed in the flood event. Flood simulation was conducted by using nationally available dataset including high resolution digital surface model and rainfall ground station data. The simulation was calibrated with discharge observation data in Batang Sinamar and gave a good result with Nash Sutcliffe Efficiency index and correlation value 0.768 and 0.908 consecutively. The result of simulation using 10-year and 25-year return showed the increasing discharge by 15.72 percent from 406.77 m3/s to 470.74 m3/s. Furthermore, the average of peak inundation water level had increased from less than 1.5 meters to more than 1.5 meters. Based on these results, it can be concluded that the model can predict the potential inundation area in Batang Sinamar River Basin in Lima Puluh Kota Regency.