The experiment results show that the microstructure control plays a key role for enhancing toughness of heavy thick X80 plates at low temperature, especially DWTT property. The toughness of heavy thick X80 plate at low temperature is not only related to the bainite grains and M/A islands, but also impacted by the original austenite grain size. Finer original austenite grain size benefits to increase the BF/AF ratio in volume of a base material enlarge the crystal orientation difference of microstructure transformation inside austenite. Cracks on a broken section of DWTT samples can (a) penetrate the coarse grains directly, (b) propagate in Zig-Zaga way in the fine grains, and (c) be around the boundary of original austenite grains. The stable and high toughness of heavy thick X80 steel plates from the mass production can be achieved at low temperature made with the reasonable chemistry, clean steel, non-defect slab technologies and OHTP rolling practice.