Langsa river that crosses the city during high rainfall often experiences flooding around the river's border. This is due to the river which is not being able to accommodate the excess discharge so that the overflow of water has a negative impact on the facilities and infrastructure around the river. In addition, flooding is also affected by the damage of Langsa watersheds which is characterized by a decrease in the ability of storing the water. There are few information available regarding the high potential impact and inundation of floods in detail caused by flood discharge plan, so that this investigation aims to determine the height and inundation of floods for a return period of 5 and 100 years as a basic information in flood disaster mitigation in Langsa City. The scope of the analysis was carried out by several methods, namely the analysis of the maximum rainfall frequency distribution with the normal distribution method, log normal, log-Pearson type III, and Gumbel, periodic analysis of planned flood discharge with the Nakayashu Synthetic Unit Hydrograph method and analysis of water level and flood inundation with the help of HECRAS 5.01 and GIS software to encounter the risk of flood inundation of the stream. The results of this study denote that the flood discharge which occurs in the river due to the maximum daily rainfall (CH) based on the Gumbel distribution results in a flood discharge (Q5) of 361.21 m3/second with a water level of 1.0 meter. Meanwhile, the flood discharge (Q100) produces 564.65 m3/second with a flood water level of 3.0 meters and 33.86 square kilometers area of indunation. Moreover, with the availability of discharge information (Q100), it is predicted that 15 villages are potentially affected by flooding. The proposed analysis has auspicious value in the practical implementation of flood disaster mitigation in Langsa City.