Cu precipitation strengthening plays an important role in the fabrication of high-strength low-alloy (HSLA) steels. The nature of Cu precipitation and the actual distributional morphology of Cu precipitates have a significant effect or directly determine the strength and toughness of HSLA steels. HSLA steel is weldable without preheat by reducing C to a low concentration. To compensate for the decrease of strength caused by reducing C, Cu was added to HSLA steel for precipitation strengthening by nanoscale Cu precipitates. The size, number density and composition of Cu nanophases could be well characterized by the atom probe tomography (APT), and the Cu nanophases obtained by APT analysis are usually termed Cu clusters. In the study, the specimens were austenitized for 30 min at 900 ℃followed by water quenching, and tempered isochronally for 60 min at different temperatures. The hardness was conducted, the microstructure and Cu precipitate were analyzed by HRTEM and APT. During tempering, Cu precipitation happened, Cu precipitate Moire fringe formed and the Cu precipitate transformed to fcc structure; the lath boundary gradually bulged out and migrated, a repeat of bulging and migration of local parts of lath boundary resulted in migration of the whole boundary, and lath martensite transformed to equiaxed ferrite finally. At 500℃, the strengthening peaked by Cu precipitates. During 400-500℃, the number of Cu clusters changed greatly when the Cu isoconcentration set at different values, this indicated that the Cu precipitates  were on the stage of nucleation; while the number of Cu clusters changed little during 500-650 ℃, this indicated that the Cu precipitates were on the stage of coarsening. The Cu, C, Mo and P segregated at the grain boundary. The boundary could provide Cu solutes and nucleation sites for Cu precipitation, leading to the segregation of Cu clusters at the grain boundary. The Ni, Mn and Al segregated at the heterophase interface between Cu precipitate and ferrite matrix forming a core-shell structure.