We conducted experiments at Fukido mangrove, northeastern Ishigaki Island, Okinawa, Japan in March 1999, to quantify the nitrogen cycle for simulations of nitrogen cycling and export. We measured concentrations of TON (total organic nitrogen), nitrate (NO3- + NO2-), ammonia (NH4+) in the creek water, and as physical parameter, measured flow rate and water level. Concentrations of TON and NO3- + NO2- were higher at high tide and lower at low tide, whereas NH4+ concentration had an opposite trend. Export flux of NO3- + NO2- was positive (62 mol.d(-1)) but export of TN (total nitrogen) was estimated -21 mol.d(-1), so that Fukido mangrove is a source of nitrogen to its coastal community. And we constructed a model of nitrogen cycle and simulated the export fluxes. Simulated TON export was only 6.6%. In the simulations, the vegetation was a net sink for nitrogen, meaning that it is string nitrogen as biomass. There is no mechanism to input into the ecosystem in this model. Nitrogen fixation may be a significant source of nitrogen to the mangrove ecosystem, and we will need to conduct more studies to clarify here its role in the nitrogen cycle.