Hyperduplex, as a new class of duplex stainless steels, having high Cr and Mo present excellent combination of mechanical and corrosion resistance, due to their strict composition control and ferrite/austenite phase balance. This balance may, however, be disturbed during welding in both the weld metal and the Heat Affected Zone (HAZ) due to the rapid cooling rates. Those may lead to loss of the good corrosion and mechanical properties of the weldments. The present investigation is to establish the effect of heat input and the nitrogen addition in the argon shielding gas, for controlling the microstructure of hyperduplex stainless steels welded by the Gas Tungsten Arc Welding (GTAW) technique autogeneously. Hyperduplex stainless steel in the form of tube having outside diameter of 32 mm and thickness of 2 mm, was welded using limited range of heat input to control the microstructure in the HAZ, and using the nitrogen addition of 2-5% into argon shielding gas to control the ferrite/austenite phase balance of the weld metal. The microstructure of the weldment was examined by calculating the volume fraction of ferrite and austenite phases. The result shows that the heat input of 0,6 kJ/mm gives the optimum ferrite/austenite phase balance in the HAZ. The addition of 2% nitrogen into argon shielding gas is recommended to give the optimum balance of ferrite/austenite phases in weld metal in addition to the heat input employed. The heat input higher than 0,6 kJ/mm promoted sigma phase at the HAZ as well as at the weld metal particularly when welded with addition of more than 2% nitrogen in the argon shielding gas.