HY-80 is the high yield steel that commonly used for naval ship and submarine. Arc welding operations are a critical stage in the fabrication of this steel. During welding, the problem may occur in the heat affected zone due to the high temperature makes the microstructure suddenly changes. Coarse grain heat affected zone (CGHAZ) develops close to the fusion line, steel become brittle and the impact toughness decrease. This research investigated the microstructure of HY-80 weldment, impact toughness at sub-zero temperatures, and hardness distribution along the cross-section of the welded joint. ER100S welding wire, Ar+10%CO2 shielding gas mixture and single V-groove butt joint with an angle of 60° were selected before welding. 12 mm thick of HY-80 steel plate that used in this experiment was multipass welded by gas metal arc welding (GMAW). Impact toughness at sub-zero temperature, hardness and microstructure evolutions of base metal (BM), heat affected zone (HAZ) and weld metal (WM) were observed. The result shows at a temperature of-80 °C; the lowest impact toughness was measured at WM (61 J) as compared to fusion line (101 J) and BM (217 J). The hardness measurement shows the maximum hardness was measured in CGHAZ followed WM and BM. Vickers hardness test result of the weld joint at the bottom area is higher than top area. It may cause of the low heat input of back weld compared to other passes. The lower heat input, the cooling rate increased and initiated the formation of a hard phase. The microstructure of WM shows acicular ferrites and non-metallic inclusions, these inclusions may deteriorate the impact toughness.