In order to understand the effect of niobium additions on the microstructures of the HAZ (heat-affected zone) in mild steels, a plain C-Mn steel (without niobium content) and three niobium containing steels with 0.01, 0.02 and 0.04 mass%Nb, respectively were investigated through simulated HAZ experiments with the heat inputs of 2 × 10 6 , 5 × 10 6 and 8 × 10 6 J / m. The microstructures of simulated coarse-grained HAZ have been examined by optical metallography and transmission electron microscopy. It was found that the addition of niobium had a significant effect on the transformation. For high energy heat inputs (8 × 10 6 and 5 × 10 6 J/m), the additions of niobium evidently retarded the pearlite formation even in the case of 0.01 mass%Nb containing steel; the microstructures of coarse-grained HAZ of niobium-containing steels consisted mainly of secondary Widmanstatten ferrite, but that of the niobium-free steel comprised a large amount of pearlite besides secondary Widmanstatten ferrite. In the condition of low energy heat input (2 × 10 6 J/m), the microstructures of coarse-grained HAZ of niobium-containing steels were all similar and consisted mainly of interlocking ferrite plates with small amounts of bainite and Widmanstatten ferrite; while that of the niobium-free steel comprised Widmanstatten ferrite and martensite with a small quantity of pearlite. The results from Charpy impact test indicated that the niobium-containing steels treated with the heat input 2 × 10 6 J/m could possess higher toughness than those treated with the heat inputs of 5 x 10 6 and 8 × 10 6 J/m. It is proposed that the interlocking ferrite structure, which forms in niobium-containing steels under 2 × 10 6 J/m heat input, improves the toughness property.