Nanoscale transformation twins (NSTTs) were observed in high-strength pipeline steels of three different grades (X80-X100) having carbon content of 0.038-0.065 wt%. The microstructure and formation of the NSTTs were characterized by field emission transmission electron microscopy. The results showed that the NSTTs were generated in two body-centered cubic microstructures-polygonal ferrite and martensite-in the martensite-austenite (MA) islands. The NSTTs were nucleated at the grain boundaries (GBs) between polygonal ferrite and bainite or triple junctions of MA islands through the emission of Shockley partial dislocation from GBs during the austenite transformation to ferrite and bainite, and extended into the interior of the ferrite grains and MA islands. The NSTTs were typical {112} -type twins. Most NSTTs displayed groups of twins. The thickness of twin plates in the groups of twins was less than 10 nm.