A dispersion model coupled with the Princeton Ocean Model was used to estimate the average residence time of the water inJiaozhou Bay. The tidal simulation agreed quite well with drift experiments and water elevation observations at the Dagang tidestation in the east coast of the bay. In particular, in situ measurements of 228Ra and salinity were carried out to calibrate thedispersion model. The modelled average residence time was about 52 days, ranging from less than 20 days in the deep part nearthe bay channel, the only passage connecting the bay to the Yellow Sea, to over 100 days in the shallow area in the northwest. Thespatial difference of average residence time was controlled by tidal residual currents and the distance to the bay channel. Themodelled tidal exchange rate was uneven in the bay, and consistent with 228Ra observations. The temporal evolution of the passivetracer accords with the evolution of the rain fraction after the rainstorm in August 2001.