It is noted that the red tides and the oxygen-deficient water mass are extensively developed in Masan Bay during summer. The nutrients mass balance was calculated in Masan Bay, using the three-dimensional numerical hydrodynamic model and the material cycle model. The material cycle model was calibrated with the data obtained on the field of the study area in June 1993. The nutrients mass balance calculated by the combination of the residual currents and material cycle model results showed nutrients of surface and middle levels to be transported from the inner part to the outer part of Masan Bay, and nutrients of bottom level to be transported from outer part to inner part of Masan Bay. The uptake rate of DIN in the box A1(surface level of inner part) was found to be 337.5㎎/㎥ ·day, the largest value in all 9 boxes and that of DIP was found to be 18.6㎎/㎥·day in box A1, and the regeneration rate of DIN was found to be 78.2㎎/㎥· day in the box A3(bottom level of inner part), and that of DIP was found to be 18.6㎎/㎥· day in box A1. The regenerations of DIN and DIP in the water column of the entire Bay were found to be 7.66ton/day and 760㎏/day, respectively. And the releases of DIN and DIP from the sediments of the entire Bay were found to be 2.86ton/day and 634㎏/ day, respectively. The regeneration rate was 2.5 times as high as the release rate in DIN, and 1.2 times in DIP. The results of mass balance calculation showed not only the nutrients released from the sediments but the nutrients regenerated in water column to be important in the control and management of water quality in Masan Bay.