Low temperature pressure vessel 08Ni3DR steel has a good strength and toughness at a very low temperature（-100 ℃）. In practical engineering application,it is one of the difficult problems to ensure the low temperature impact toug-hness of welded joints in the process of pressure vessel man-ufacturing. The determination of the weakest zone and the effects of the weakest zone are very important for the characterization of the actual welded joints. The microstructures and toughness of the welded joints of 08Ni3DR pressure vessel steel were systematically studied by preparing the Charpy V-notch on the base metal,weld metal and different locations of heat-affected zone. The results show that the weakest toughness zone of the welded joint is the coarse grain heat-affected zone,which is a composite microstructure composed of coarse granular bainite and lath bainite. As the proportion of the width of coarse grain heat affected zone in the front of the notch tip increases,the impact absorbed energy of the specimen decreases. When the proportion of the width of the coarse grain heat-affected zone reaches 100%,the impact absorbed energy is 27 J,and the impact toughness of the coarse grain heat-affected zone reduces by 90.7%. Above two aspects fully show that the weakest zone has a great effect on the impact toughness of the welded joint.