Abstract: The low carbon-silicon-manganese alloyed billets with 0.10% niobium instead of molybdenum were rolled to be the X80 pipeline steel via HTP (high temperature processing) in lab, of which all the mechanical properties came up to the standard API Spec 5 L. The main parameters in rolling schedule were as follows: finishing temperature 830~850°C; end cooling temperature 500~550°C, cooling rate 25~28°C/sec; total percent reduction in finish rolling not less than 75%. It was found that the microstructure of the steel plate thus rolled is mainly composed of acicular ferrite with small parts of massive ferrite and granular bainite, and the island structure is found on grain boundary. Such a structure can provide high tensile strength and impact toughness. Metalloscopy, SEM and EDS were applied to observing the impact fracture and it was found that the brittle 2nd-phase particles and prior austenite grain boundaries are the main cracking sources. So, controlling the brittle 2nd-phase particles and avoiding the prior austenite grain boundaries will improve the toughness of the pipeline steel efficiently.