Small-Angle Neutron Scattering (SANS) allows characterization, in a non-destructive way, of the heterogeneities (precipitates, pores, cavities,…) ranging in size between 1 and 100 nm. The SANS technique has been used in order to study the microstructural evolution of martensitic steels under neutron irradiation at 325 °C up to a dose of 0.9 dpa, and after thermal aging between 250°C and 550°C up to 22 000 hours. The investigated materials were Fe-7.5/1 1Cr WVTa Reduced Activation and Fe-9/12Cr MoWV conventional martensitic steels. SANS experiments were coupled with Transmission Electron Microscopy (TEM) observations. The structural information obtained by SANS and TEM is compared with the evolution of mechanical properties. Three main results have been obtained: (1) above 8.8 at.% Cr in solid solution, the ferritic matrix is unstable at an aging temperature of 400°C and separates into a Cr-depleted α and a Cr-rich α′ b.c.c. phase; (2) moreover, a strong radiation-accelerated or induced α′precipitation is observed in alloys irradiated at 325°C; (3) the precipitation of Laves phase Fe2(Mo,W) is strongly dependent on the Mo and W content in the alloys.