Gajah Wong River is one of the rivers that crosses Sleman Regency, Yogyakarta City and Bantul Regency which in flooded in 2017. Therefore it is necessary to analyze the full bank capacity which aims to analyze the ability of the river to accommodate the water flow that passes through the river. The analysis of the full bank capacity of the Gajah Wong River was carried out by RMA2 simulation on the AQUAVEO 10.1 SMS Software. In this study, river and riverbank baseline modeling, downstream water level elevation and flowrate were obtained from the results of trial and check to obtain the water level along the river that corresponds to the full bank capacity. The value of the full bank capacity is then matched with the return data obtained from the Ministry of Public Works of the Serayu-Opak River Basin (KPU-BBWS). Based on the simulation results of RMA2, the flowrate that has the same water level elevation and river cliff elevation is obtained, which is 5 m³ / s, 6 m³ / s, 8 m³ / s, 10 m³ / s, 11 m³ / s and 12 m³ / s. Based on the discharge data it is known that the Gajah Wong River has not yet entered its return period, because the largest discharge obtained from the simulation results is 12 m³ / s which is smaller than the smallest return value at the 2-year return period of 85.77 m³ / s.