The　continuous　liquid　phase　synthesis　of　nickel　phosphide　(Ni2P)　nanoparticles　was　studied　in　a　helically　coiled　tube　(HCT)　reactor　both　in　single-phase　and　two-phase　slug　flows.　The　reactants　were　nickel　acetylacetonate　and　tri-n-octylphosphine　(TOP)　in　a　1-octadecane　solvent.　For　the　single-phase　mode,　various　parameters　such　as　reaction　temperature,　residence　time,　TOP　concentration,　and　P/Ni　ratio　were　studied.　It　was　found　that　lower　temperatures　of　320　and　340　°C　resulted　in　the　formation　of　mixed　Ni12P5　and　Ni2P　phases,　while　a　higher　temperature　of　360　°C　gave　mostly　Ni2P,　with　particle　sizes　increasing　from　28　to　42　nm.　Upon　varying　the　contact　time　between　88　and　340　s　at　360　°C,　a　likely　sequence　of　reaction　involved　an　amorphous　phase　that　was　transformed　in　parallel　to　Ni,　Ni12P5,　and　Ni2P.　When　the　flow　in　the　HCT　was　changed　to　a　two-phase　slug　flow　by　using　N2　to　split　the　continuous　liquid　phase　into　small　liquid　columns,　the　product　was　nearly　100%　Ni2P　and　the　particle　size　was　as　small　as　3-4　nm.　This　was　attributed　to　the　enhanced　mass　transfer　in　the　small　liquid　columns　of　the　slug　flow　that　led　to　higher　reaction　rates.　It　is　highlighted　that　the　HCT　operated　in　a　slug　flow　is　an　efficient　and　continuous　method　for　the　fabrication　of　nanoparticles.　©　2020　The　Royal　Society　of　Chemistry.