Anthropogenically-induced changes of the thermohaline circulation could be potentially important in future climate regimes. By employing a coupled GCM (DOE's PCM), the maximum Atlantic thermohaline circulation weakens by about 3 Sv or 10% in an idealized forcing (1% CO2 increase) transient climate experiment at time of CO2 doubling compared with the control run. The weakening of the THC is accompanied by reduced poleward heat transport in mid-latitudes. Further analysis indicates that the weakening of the THC seems mainly caused by the freshening of the surface ocean in the northen North Atlantic region. However, variation of the deep convection intensity in this region is not uniform. In general, it shows a significant weakening of the deep convection in the Labrador Sea region and a mild strengthening of the deep convection at the south of the Denmark Strait region. Because increased CO2 induces warming and freshening effects, the density of the surface water is reduced when it reaches the Labrador Sea. Thus the intensity of deep convection there is weakened. Conversely, increased salinity at the south of Denmark Strait intensifies the deep convection there.