Abstract: The irradiation damage behavior of two China low activation martensitic （CLAM） steels with different composition was studied by high voltage electron microscope. The results indicate that a lot of voids form in CLAM steel without silicon addition during electron irradiation. When irradiated to 14 dpa at 450 ℃, the number density of voids is about 8.7×10²¹ m^－3, and the irradiation swelling is approximately 0.26%. The irradiation swelling of CLAM steel without silicon addition at 450 ℃ is larger than that of at 500 ℃. When the irradiation damage rate is 2×10－3 dpa/s, no voids can be found in CLAM steel with silicon addition, so silicon is a good alloying element for irradiation swelling resistance in CLAM steels. When irradiated at 450 ℃, a kind of coherent precipitates form in silicon added CLAM steel.