Graphitic　carbon　nitride　(g-C3N4)　is　a　novel　two-dimensional,　nitrogen-doped　carbon　material　with　various　applications　in　many　fields.　Recently,　g-C3N4　has　exhibited　excellent　ability　in　separations.　In　this　work,　mechanisms　of　water　and　ion　permeation　across　nanoporous　g-C3N4　membranes　with　different　pore　diameters　were　studied　using　non-equilibrium　molecular　dynamics　simulations.　The　water　conduction　rates　derived　from　our　simulations　are　in　good　agreement　with　previous　experimental　data.　Due　to　the　nanoscale　confinement,　we　observe　a　stacked　water　cluster　structure　within　the　narrowest　nanochannel.　Similar　structures　are　obscure　as　the　increase　of　the　nanochannel　sizes.　For　desalination　purposes,　we　find　that　the　nanoporous　g-C3N4　membrane　with　the　desired　opening　may　completely　reject　calcium　and　chloride　ions,　whereas　sieve　out　monovalent　cations　with　acceptable　rejection　rates　(over　70%).　This　work　provides　molecular　insight　into　the　mechanism　of　water　and　ion　transport　through　nanoporous　g-C3N4,　which　strongly　supports　that　this　novel　material　is　a　promising　candidate　for　developing　selective　membrane　for　water　desalination.　©　2018