Abstract: Irradiation generally can induce the formation and change of nano-scale clusters or precipitates in RPV steel. Small angle neutron scattering（SANS） technology is one of the important tools to study such nano-structures. SANS has many technical advantages, such as a suitable gauge dimension, strong penetrability, sensitivity to magnetic structure, and ability to distinguish adjacent elements. This technology can be used to study the size, composition, and volume fraction of the nano-structure induced by neutron irradiation, and further understand the macroscopic mechanical properties of RPV steel. In this paper, several typical application cases are introduced, including the study of nano-structure observation in a magnetic field environment; the characteristics of irradiation defects under the aspect of long term operation; the effects of material composition and the irradiation dose rate on the size and volume fraction of precipitated clusters; the evolution of nano-precipitation structures during in-situ annealing; and the effect of hydrogen on nano-clusters.