The　key　to　improving　nanofiltration　membrane　permeance　is　reducing　its　thickness　while　maintaining　high　rejection.　Herein,　a　25　nm　thick　ultrathin　polyamide　layer　was　prepared　by　a　microphase　diffusion-controlled　interfacial　polymerization　(MDC-IP)　of　poly(ethyleneimine)　and　trimesoyl　chloride,　which　is　much　thinner　than　the　conventional　interfacial　polymerization　(CIP)　polyamide　layer.　A　new　formation　mechanism　for　such　an　ultrathin　layer　is　presented,　which　included　a　microphase　interfacial　reaction　and　eliminated　loose　layers　due　to　the　confinement　of　microphase　diffusion　and　the　termination　of　stepwise　diffusion.　Moreover,　the　polyamide　layer　was　post-cross-linked　to　form　a　stable　dual-cross-linked　interwoven　structure.　Such　a　membrane　showed　an　ultrahigh　permeance　of　1246　kg/(m(2)　h　MPa),　which　was　23　times　that　of　CIP　membranes.　MDC-IP　could　efficiently　control　the　microinterface　between　two　immiscible　phases,　which　provided　a　facile　way　to　regulate　the　membrane　at　nanoscale.