Two reduced activation ferritic/martensitic (RAFM) alloys with chemical compositions guided by thermodynamic modelling were designed, fabricated, and mechanically tested in previous work. The experimental alloys demonstrated significantly refined grain and subgrain size with a high density of dislocations leading to higher yield and tensile strength with respect to EUROFER97–2 without sacrificing the impact toughness. This improvement has been achieved thanks to the dedicated chemical and thermal treatment that alternates the nucleation, growth and coarsening as well as enables the presence of high-density small precipitates as compared to EUROFER97 steel. In this work, TEM investigation was performed to quantitatively describe the microstructural parameters such as the average sizes of M 23C 6 and MX particles, number density of the particles and dislocation density. Furthermore, the strengthening contribution of each microstructural feature was calculated, and the correspondence of the total strength derived via microstructure to the measured yield strength is discussed.