Abstract: The effect of shielding gas flow (SGF) on weld microstructure was investigated in the plasma arc welding (PAW) of pure N6 Ni without filler wire. The results show that weld appearance has an obvious metallic luster and no surface defects with the increase of the SGF. The HAZ width on the back side of weld seam increases with the SGF rate being increased, and reaches gradually up to that on the front of weld seam. The grains size increase from the base metal to the center of weld. The grain size in HAZ has no few variations with the increase of the SGF, while the grains in the weld center become gradually coarser. Meanwhile, a large number of pores appear when the shielding gas is not enough. From analysis of SEM and EDS results, there are a large number of oxides with Ni, C, N, and Mg as main components in the inner wall of the porosities in weld, and oxygen-riched impurities groups have a texture of cerebral cortex like.