A multiphase calcium alginate membrane composited with cellulose nanofibers (CNF) was successfully prepared by using the method. The maximum stress and strain, water content, water permeability, and mass transfer characteristics were examined according to the addition ratio of CNF. Increasing of the amount of CNF enhanced the maximum stress and reduced the maximum strain, thereby suggesting that CNF impart flexibility and stiffness to the composite membrane. The CNF in the calcium alginate noticeably increased the water flux at the same pressure. The permeability of the composite of the CNF membrane was higher. The effective diffusion coefficient dramatically decreased 2.5 × 10 3 times when the molecular weight increased tenfold (from 60 to 604 Da). Moreover, stronger dependency was observed in the CNF composite membrane. The morphology of the cross-section of the membrane was observed using scanning electron microscopy. The appearance of the CNF composite membrane became rougher when the amount of CNF increased. Keywords Cellulose nanofiber • Membrane • Mechanical strength • Effective diffusion coefficient • Water permeability List of symbols A c Initial membrane cross-sectional area (m 2) A m Effective area of the membrane (m 2) B max Maximum breaking load (N) C fi Initial concentration in the feed solution (mol/L) C s Concentration in the stripping solution (mol/L) D Diffusion coefficient estimated from an empirical equation in the bulk aqueous phase (m 2 s −1) D eff Effective diffusion coefficient (m 2 s −1) d Diameter of glass petri dish (m) H V Volumetric water content of membrane, defined by Eq. (4) (-) J V Volumetric water flux (m 3 water m −2 area s −1) K OL Overall mass transfer coefficient (m s −1) k m Membrane mass transfer coefficient (m s −1) L Length of membrane at break (m) L i Initial length of membrane (m) L p Water permeability coefficient (m 3 water m −2 area m −1 thickness Pa −1 s −1) l m Membrane thickness (m) * Ryo-ichi Nakayama,