In　this　study,　carbon　nanofibers　(CNFs)/epoxy　composites　incorporating　different　contents　of　CNFs　were　fabricated　and　immersed　in　distilled　water　for　180　days.　Water　absorption　test,　tensile　tests,　scanning　electron　microscopy　(SEM)　observation,　dynamic　mechanical　analysis　(DMA),　and　Fourier　transform　infrared　(FTIR)　spectroscopy　were　performed　to　evaluate　the　effect　of　CNFs　content　on　hygrothermal　aging　behavior　of　CNFs/epoxy　composites,　and　to　reveal　the　hygrothermal　aging　mechanism　of　such　composites.　Test　results　showed　that　the　addition　of　CNFs　into　pure　epoxy　resin　could　obviously　improve　its　hygrothermal　aging　behavior.　The　CNFs/epoxy　composites　had　better　resistance　to　water　absorption　than　pure　epoxy　because　CNFs　improved　the　water　barrier　properties　of　the　composites.　The　improvements　of　glass　transition　temperature　and　the　storage　modulus　of　the　composites　before　and　after　aging　were　attributed　to　the　fact　that　the　addition　of　CNFs　limited　the　loosening　of　molecular　chains　and　increased　the　second　cross-link　density　of　epoxy　resin.　FTIR　spectroscopy　showed　that　the　number　of　-OH　functional　groups　of　CNFs/epoxy　composites　was　decreased,　which　revealed　that　CNFs　inhibited　the　hydrolysis　of　epoxy　resin　and　water　absorption.　This　study　demonstrates　that　adding　appropriate　amount　of　CNFs　into　epoxy　resin　has　potential　to　improve　the　long-term　durability　of　epoxy-based　composites　in　hygrothermal　environment.　(c)　2021　Elsevier　Ltd.　All　rights　reserved.