Abstract Kaohsiung Harbor is one of the most important international sea ports in the world. Due to the long-standing lack of in-situ current data, the complex variations of the flow field in this basin still remain unclear. As a consequence, the related environmental problems such as oil spills, water quality management and ship maneuvering safety , have long been a great concern in this harbor. The present study is conducted to better understand the flow field in the Kaohsiung Harbor. A series of synoptic flow observations of the Kaohsiung Harbor were conducted by using towed-ADCP or EM current meters. From these observations it can be shown that the flow field of the Kaohsiung Harbor is¡GWater entering the harbor through the second entrance and exiting the harbor through the first entrance during ebbs. During floods the flows are reversed. A 3-D numerical model (from POM) is developed for the Kaohsiung Harbor. The flows are more complicated by the M2 tide driven than by the mixed tide driven. From the results by the M2 tide driven show the ocean current is variable, especially the south ocean current. Therefore, the flows are more complicated owing to the mixed tidal characteristics and shoreline geometry. The maximum current speeds amount to 30 - 40 cm/s in the narrow first entrance and 10 - 20 cm/s in the second entrance. It is clearly demonstrated from the model results that drainage from the Chien-Chen River affects greatly the salinity and circulation patterns of the Kaohsiung Harbor, causing the salinity of the first entrance to be lower than that of the second entrance, and the surface layers flowing outward toward the ocean while the lower layers displaying tidal oscillations. From the modeling results, the influence of the wind on the harbor flows is insignificant and the tide is main force in the harbor. Under the simultaneous forcing of river and wind, flood and ebb tidal streams leaving the two entrances are found to diverge in a flow stagnation area inside the harbor near Pier 45 and 61, respectively. Based on the modeling results, it can be concluded that the main factors affecting the flow patterns of the Kaohsiung Harbor are (1) mixed tidal nature, (2) shoreline geometry and (3) river runoff