In this study, the effect of bacterial cellulose nanofibers (BCNF) incorporation on the structural and rheological properties of casein gels was investigated, where the mixed BCNF and casein gels were prepared by adding gluconic acid δ-lactone (GDL) to acidify the mixed polymer solutions at 3.0% casein concentration (w/v) and varying BCNF concentrations (0–0.5%, w/v). By changing the addition amount of GDL, the mechanical and structural properties of the mixed gels were studied at above, near and below the electric point (pI) of the casein. At pH above the pI of the casein, the introduction of BCNF initially increased the gel strength, but further addition of BCNF weakened the mixed gels. At near and below the pI of the casein, the incorporation of BCNF continuously increased the gel strength. Besides, all gels showed good structural homogeneity, without macroscopic phase separation occurring, which indicated good compatibility of BCNF with the casein gels.