Cu-Ni-Si　alloys　have　been　widely　applied　in　electronic　and　electrical　industries.　The　precipitation　behavior　of　some　of　the　Cu-Ni-Si　alloys　is　still　not　well　understood.　In　this　study,　the　precipitation　behavior　of　the　Cu-3.0Ni-0.72Si　alloy　aged　at　600　degrees　C　for　different　times　was　investigated　by　transmission　electron　microscopy,　atom　probe　tomography　and　phenomenological　theory　of　precipitation　crystallography.　A　new　orientation　relationship　(OR)　between　the　precipitates　and　the　Cu　matrix　was　found　in　the　over-aged　condition　and　a　coarsening　mechanism　of　the　metastable　precipitates　was　put　forward.　The　two-　and　three-dimension　invariant　line　theories　were　successfully　applied　in　interpreting　the　evolution　of　the　ORs　and　the　morphologies　in　the　Cu/delta-Ni2Si　(Cu/delta)　system.　At　the　early　stage　of　aging,　the　fine　metastable　delta＇-(Cu,　Ni)(2)Si　precipitates　are　coherent　with　the　Cu　matrix,　with　a　quasi-Bain　OR　of　(110)(Cu)parallel　to(100)(delta)＇,　and　[001](Cu)parallel　to[001]delta＇,　and　four　pairs　of　parallel　conjugate　planes:　((11)　over　bar1)(Cu)parallel　to((3)　over　bar　01)(delta)＇,　(111)(Cu)parallel　to(301)(delta)＇,　((1)　over　bar　11)(Cu)parallel　to(021)(delta)＇,,　and　(1　(11)　over　bar)(Cu)parallel　to(0　(1)　over　bar1)(delta)＇.　The　precipitates　have　a　delta-Ni2Si　structure,　with　some　Ni　atoms　substituted　by　Cu　atoms.　During　growth,　the　core　region　of　the　metastable　delta＇-(Cu,Ni)(2)Si　precipitate　transforms　into　stable　delta-Ni2Si,　with　a　quasi-NW　OR　of　((1)　over　bar　11)(Cu)parallel　to(021)delta　and　[110](Cu)parallel　to[100](delta),　while　a　layer　of　metastable　delta＇-　(Cu,　Ni)(2)Si　still　exists　around　the　core.　With　prolonging　aging　time,　the　delta-Ni2Si　precipitates　with　the　OR　of　((1)　over　bar　11)(Cu)parallel　to(021)(delta)　and　[110](Cu)parallel　to[100](delta)　grow　two-dimensionally　to　form　a　plate-like　shape,　while　those　with　the　OR　of　(111)(Cu)parallel　to(301)delta　and　[(1)　over　bar　10](Cu)parallel　to[010](delta)　grow　one-dimensionally　to　form　a　fiber-like　shape.　(C)　2019　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.