Polyethersulfone　(PES)　ultrafiltration　membranes　were　prepared　by　wet　phase-inversion.　Formation　mechanism　of　pore　structure　in　the　membranes　was　studied.　For　the　formation　of　macrovoids　in　membrane　sublayer,　a　new　three-step　mechanism　was　proposed　as　follows:　(1)　the　initial　step　of　macrovoids,　forming　polymer-poor　nuclei;　(2)　the　forming　step　of　macrovoid　top　outline,　forming　domelike　top　outline　in　macrovoids;　(3)　the　growing　step　of　macrovoids,　the　boundary　renewal　of　polymer-poor　nuclei.