In order to study systematically the notch toughness and microstructural alterations across the heat-affected zone (HAZ) as functions of position and weld conditions, a unit HAZ concept was applied to a quenched and tempered SA 508 Cl. 3 reactor pressure vessel steel multipass weld. The unit HAZ, which consists of the width of HAZ (W_H) and the width of interpass (W_I), could be determined by the physical constants and welding variables of the materials. Weld HAZ thermal cycles, which correspond to the various HAZ regions, were simulated with a Gleeble 1500 thermal simulator. Seven typical positions were selected to evaluate the spatial distribution of notch toughness and microstructure in the unit HAZ. Three coarse-grained regions and two fine-grained regions showed relatively good toughness. On the contrary, an intercritically reheated region and a subcritically reheated region showed lower toughness than the base metal. The causes for the variations in toughness were discussed with reference to the microstructures and thermal cycles.