The　effective　delayed　neutron　fraction　(βeff)　plays　an　extremely　important　role　in　the　dynamics　behavior　of　the　reactors.　In　molten　salt　reactors　(MSRs),　because　of　the　adoption　of　the　fluid　fuel,　the　calculation　of　the　βeffis　different　with　that　in　the　traditional　solid-fuel　reactor.　This　work　concentrates　on　the　calculation　of　the　βeffin　MSRs.　The　βeffof　the　MOSART　was　calculated　and　the　effect　of　external-loop　transit　time　and　inlet　velocity　on　the　βeffis　analyzed　in　detail　in　this　paper.　The　results　indicate　that　the　increasing　of　the　external-transit　time　decreases　the　βeff,　and　the　increasing　of　the　inlet-velocity　also　decreases　the　βeff.　©　2017,　Editorial　Board　of　Journal　of　Nuclear　Power　Engineering.　All　right　reserved.