High-strength steel HY-80 is widely used for ship and submarine construction. This steel has a good combination of strength, ductility, and toughness at room temperature and sub-zero temperatures. Fusion welding is a very important process in high-strength steels, but the welding problem appears in a heat affected zone (HAZ) near the fusion line where a coarse grain heat affected zone (CGHAZ) is formed causing the impact toughness in CGHAZ reduce drastically so that the weld joint become brittle. CGHAZ is more susceptible to hydrogen-assisted cracking than weld metal (WM). The objectives of this study were to evaluate changes in microstructure, hardness distribution mode, and grain growth in weld joint. The material used in this research is high strength steel HY-80 welded with gas metal arc welding (GMAW) method using 90% Ar + 10% CO2 shielding gas and ER100S welding wire. Both butt and fillet weld joint types were applied. The results show the highest hardness was observed at CGHAZ for both butt and fillet weld. Butt weld joint type shows the hardness of HAZ and WM near bottom surface are higher than near upper surface due to lower heat input so that it contributed to higher cooling rate and the formation of martensite occurred.