Abstract: The relationship among the inierostructure, precipitation behavior and mechanical properties of China low activation martensitic (CLAM) steel was studied by optical microscopy, transmission electron microscopy (TEM) and chemical phase analysis. It is shown that the quenched microstructure is martensite, but after being tempered at 760℃ the mierostrueture transforms into finer and more homogeneous sorbite. The tensile strength, yield strength and elongation of the steel at room temperature are 697 MPa, 652 MPa and 24.4%, respectively; however, at 600℃ their values are 453 MPa, 452 MPa and 23%, respectively. The ductile-brittle transition temperature (DBTT) is -60℃. The second phase particles, sized from 30 to 70 nm, mainly are M₂₃C₆ and Ta(C, N) with FCC crystal structures. The particles, mostly distributed at grain boundaries but few dispersively in grains, can produce precipitation strengthening, as the key strengthening mechanism of the steel.