Abstract: In the light of the complexity in design for the Wang Noi Kaent Koi Natural Gas Transmission Pipeline in Thailand, the metal composition of API 5L X65 steel pipe is analyzed. The result shows that all composition indexes of this steel pipe meet the needs of application in sour environment. To avoid the failure of pipe in sour environment, the authors consider that the candidate welding stick and welding process should be taken into account, and meanwhile the human injury problem from X-ray inspection to some sensitive zone also avoided or eliminated by effective methods or measures.