The microstructure of the welded joint in a microalloyed steel was varied by using several filler materials, heat inputs and welding methods. The effect of microstructure on the weld metal and HAZ notch toughness at -60 °C was investigated. The microstructural factors that characterise the weld metal and HAZ notch toughness in the best way, were determined. The most favorable weld metal and HAZ microstructure was fine grained with negligible amounts of coarse grain boundary zones and through the grain growing laths. In the weld metal the fine grained microstructure consisted of acicular ferrite and normalisized weld metal. The pro-eutectoid zones at the grain boundaries showed a harmful effect on the notch toughness. In the coarse grained HAZ the lower bainitic microstructure had the smallest grain size and hence the best toughness. The notch toughness was decreased by pro-eutectoid ferrite and apparently by alloy carbide precipitation. The highest values for HAZ toughness in a microalloyed steel were achieved by using low heat input without weaving. In the weld metal the best toughness was obtained by the nickel-alloyed filler material. The manganese-silicon-alloyed submerged arc filler material with a Mn/Si-ratio of 3,0 was also found suitable for low temperature (-60 °C) use as far as the heat input did not exceed 25 kJ/cm.