The studies of the rare earth elements (RE) in low carbon steels suggest that the RE inhibits the ferrite transformation, which is the same effect as Mo alloying in pipeline steels. The purpose of this work is to discuss the relationships between the RE microalloying and the microstructure in pipeline steels. The X80 pipeline steels with different RE and Mo additions have been produced by vacuum induction furnace. The Gleeble-2000 thermal simulator, optical microscopy, and scanning electron microscopy with EBSD have been used. The continuous cooling transformation (CCT) curve was obtained and analyzed, combined with the study of microstructure. The results indicate that the microstructure of thermal simulator test metal is characteristic of quasi-polygonal ferrite and bainite, and trace RE could significantly inhibit the transformation of quasi-polygonal ferrite. The 0.0040wt% content of RE plays the same role as 0.1 wt% content of Mo alloying in pipeline steels. What’s more, the fine bainite grained structure is obtained with RE microalloying. Theoretically RE could be employed in pipeline steels as microalloying, and a partial substitution of Mo by RE is possible. The studies of the rare earth elements (RE) in low carbon steels suggest that the RE inhibits the ferrite transformation, which is the same effect as Mo alloying in pipeline steels. The purpose of this work is to discuss the relationships between the RE microalloying and The microstructure in pipeline steels. The X80 pipeline steels with different RE and Mo additions have been produced by vacuum induction furnace. The Gleeble-2000 thermal simulator, optical microscopy, and scanning electron microscopy with EBSD have been used. The continuous cooling transformation (CCT ) curve was obtained and analyzed, combined with the study of microstructure. The results that that microstructure of thermal simulator test metal is characteristic of quasi-polygonal ferrite and bainite, and trace RE could significantly inhibit the transformation of quasi-polygonal ferrite. 0.0040 wt% content of RE plays the same role as 0.1 wt% content of Mo alloying in pipeline steels. What’s more, the fin Theoretically RE could be employed in pipeline steels as microalloying, and a partial substitution of Mo by RE is possible.