Abstract: The porosities were observed in laser penetration welding of 5 mm thick 316LN plat with butt joint. In order to investigate the porosity formation, the shielding gas flow, shielding gas type and base metal had been changed in the experiments. As well as the experimental results were analyzed by means of SEM, LM, X-ray, WEDM and analysis of chemical component of base metal. The results showed that the porosities in the weld are round and often accompanied by inclusions, and they are mainly hydrogen porosity. And for the chemical compositions of the No.1 material, the δ crystallization mode has higher susceptivity of porosity. It was confirmed that the δ phase appeared in the metallographic microstructure. In addition, impurity argon is another important factor for the porosity formation.