In order to explore the effect of silicon (Si) content on the irradiation behavior of ferritic/martensitic (F/M) steel, 9Cr F/M steel with Si of 0, 0.4%, 0.7%, and 1.0% in weight percent were prepared. All speci-mens were irradiated with 2.4 MeV iron ions at 550 degrees C to the fluence of 6.41 x 10 16 ions cm -2, corre-sponding to a peak damage dose of 70 dpa. Transmission electron microscopy (TEM) was used to charac-terize the microstructure of the alloys before and after irradiation, and nanoindentation tests were used to assess the irradiation hardening. The addition of Si inhibited the growth of both M 23 C 6 and MX pre-cipitates and promoted the nucleation of MX precipitates, and the nucleation of the M 23 C 6 precipitates was inhibited when the added Si was less than 0.7 wt.%, indicating that the precipitates can be regu-lated by Si content. The martensite lath width and the packet size of 0.4 wt.% Si content alloys are the largest and then decrease with the increase of Si content. As the Si content increases, the average size of dislocation loops decreases while the number density increases. Alloys without Si have the smallest void number density, followed by alloys with 0.7 wt.%, 0.4 wt.%, and 1.0 wt.% Si content. The hardness of the unirradiated alloy increased with the increase of Si content. After irradiation, the most severe radiation hardening occurred in the 0.4 wt.% Si alloy and the lowest in the 0.7 wt.% Si alloy.(c) 2022 Elsevier B.V. All rights reserved.