Herein, 4Cr5MoSiV1 steel samples are fabricated via selective laser melting (SLM), and the effect of tempering on microstructure and mechanical properties is investigated. Results show that tempering of SLM-fabricated 4Cr5MoSiV1 steel samples involves martensite decomposition, retained austenite transformation, and carbide precipitation, thus producing stable ferrite and alloy carbides. Post tempering, the sample's grain morphology disappears, and its grain size increases. In addition, the sample's microhardness and ultimate strength decrease, whereas its elongation increases. Post tempering at 450 ℃, fine and uniformly distributed carbides precipitate, contributing to precipitation strengthening and causing secondary strengthening of the sample. The sample's elongation after tempering twice at 600 ℃ increases to 18.6%. The fracture of unheated and low-temperature tempered samples is attributed to a brittle cleavage fracture. The fracture of the medium-temperature tempered sample is found to be a quasi-cleavage fracture. Finally, the fracture of the high-temperature tempered sample and twice-tempered sample exhibit a quasi-cleavage fracture dominated by ductile fracture.