To investigate temporal and spatial variations of hydrodynamics and environmentalconditions in Gyeonggi Bay, including Han River estuary, numerical experiments wereperformed using 3-dimensional fine grid numerical ocean model. The model successfullyreproduced the physical phenomena already known in Gyeonggi Bay where tide and freshwater discharge are dominant forcings.The calculated harmonic constants of tide and tidal current agreed well with those ofobservations at nine tide stations and two tidal current stations. Tidal asymmetries along theYeomha Waterway, mainly caused by non-linear effect, were well reproduced and agreed wellwith observations. Time series of salinity at four stations(A, B, C and D) and horizontaldistributions of monthly averaged salinity show that Gyodong and Seokmo Waterways play animportant role in fresh water discharge into the Gyeonggi Bay rather than Yeomha Waterway.