Abstract: Studied here in the essay are the different roles and mechanisms of the micro-alloy elements （i.e.，Nb，V and Ti） in the chemical composition system of the as-normalized seamless steel linepipe. Also analyzed is how chemical elements like C and N，etc. affect the micro-alloying result. Accordingly，the facts as following are revealed. In the chemical composition system of the said linepipe，elements Nb，V and Ti can get the grains well refined，and bring about the segregation strengthening effect so as to enhance strength and toughness of the steel.Increasing N content can remarkably enhance the segregation-strengthening and grain refining-strengthening effects by Element V. And reducing C content can obviously enhance impact toughness of the steel，whereas with the same carbon equivalent，the strength might be decreased.