Tidal exchanges of sea water are studied by using drogue experiments and tidal current measurement data in Gamag Bay which has two channels. At the spring tide, the volume of tidal transport in the bay was estimated to be 46∼52% of the total volume of sea water in Gamag Bay, 7.1 10$^$8/㎥. The tidal transport through the wide channel occupies 87% of the total tide transport of the bay. Residual current was deduced to flow north-northeastward at the rate of 3.254 10$^$5/㎥ per tidal cycle. the tidal exchange of the sea water during the flood flow was estimated to be approximately 26% of the tidal transport, while that during the ebb flow was 41%. The tidal exchange through the wide channel during the flood flow occupies 77% of total tidal exchange of the bay through both channels, whereas that during the ebb flow does 88%. The diffusion coefficient of 2.08∼ 2.30 10$^$7/$ extrm{cm}^2$/sec at the narrow channel was greater than that at the wide channel which was 1.2∼2.8 10$^$6/$ extrm{cm}^2$/sec.