Cu-based　alloys　can　be　used　as　a　selective　laser　melting　(SLM)　material　for　advanced　engineering　applications,　such　as　aerospace,　5G　mobile　networks,　and　high-speed　transportation.　The　mechanical　properties　and　solidification　microstructures　of　Cu　alloys　prepared　using　the　casting　technique　differ　from　those　prepared　using　the　SLM　technique,　and　SLM-built　alloys　can　involve　more　complex　microstructures　and　phase　transformations　developed　in　micromolten　pools　produced　by　high-power　laser　beams.　However,　nonequilibrium　solidification　microstructures　and　mechanical　properties　of　SLM-built　Cu-Sn　alloys　have　seldom　been　studied　in　the　literature.　In　this　work,　the　Cu-5%Sn　alloy　was　investigated　using　the　SLM　technique,　along　with　cast　Cu-Sn　alloys　for　comparison.　The　high　quality　Cu-based　alloy　samples　were　fabricated　using　the　SLM　technique,　with　optimized　processing　parameters　of　160　W　laser　power,　300　mm·s−1　scanning　speed,　and　0.07　mm　line　spacing.　The　samples　exhibit　a　relative　density　of　99.2%,　and　virtually　no　pores　and　spheroidizing　phenomena　or　warping　defects　were　observed.　The　microstructural　analysis　of　SLM-built　Cu-5%　Sn　alloy　reveals　a　nonequilibrium　solidification　feature　under　high　cooling　rates　and　rapid　alternative　thermal　conditions　during　the　SLM　fabrication　process,　in　which　the　α-Cu(Sn)　solid　solution　is　the　major　phase　along　with　γ　and　δ　phases.　Columnar　grains　and　reticular　microstructures　dominate　the　solidified　SLM-built　alloy,　while　segregated　Sn　appears　in　the　boundaries　of　all　levels　within　the　alloys.　The　Sn-rich　nanoparticles　with　super-lattice　structures　precipitates　along　the　grain　boundaries　and　inside　the　grains.　With　the　combined　effects　of　grain　fining,　super-lattice-structured　nanoparticles　precipitation,　solid　solution,　and　thermal　residual　stress,　the　SLM-built　Cu-5%Sn　alloy　shows　significantly　enhanced　mechanical　properties,　such　as　HV　133.83　Vickers　hardness,　326　MPa　yield　strength,　387　MPa　tensile　strength,　and　22.7%　fracture　extension.　Such　scientific　information　is　very　useful　for　improving　the　alloy　composition　design　and　optimizing　the　SLM　processing　parameters.　©　2021,　Science　Press.　All　right　reserved.