To understand the charging and aggregation of cellulose nanofibers (CNFs), we performed the following experimental and theoretical studies. The charging behavior of CNFs was characterized by potentiometric acid-base titration measuring the density of deprotonated carboxyl groups at different KCl concentrations. The charging behavior from the titration was quantitatively described by the 1-pK Poisson-Boltzmann (PB) model for a cylinder. The electrophoretic mobility of CNFs was measured as a function of pH by electrophoretic light scattering. The mobility was analyzed with the equation for an infinitely long cylinder considering the relaxation of the electric double layer. Good agreement between experimental mobilities and theoretical calculation was obtained by assuming a reasonable distance from the surface to the slipping plane. The result demonstrated that the negative charge of CNFs originates from the deprotonation of β(1-4)-d-glucuronan on the surface. The aggregation behavior of CNFs was studied by measuring the hydrodynamic diameter of CNFs at different pH and KCl concentrations. Also, we calculated the capture efficiencies of aggregation, using interaction energies of perpendicularly and parallelly oriented cylinders. The interaction energies between cylinders in both orientations were obtained by the Derjaguin, Landau, Verwey, and Overbeek theory, where the electrostatic repulsion was calculated from the surface potential obtained by the 1-pK PB model. From comparison of the theoretical capture efficiency with the measured hydrodynamic diameter, we suggest that CNFs can be aggregated in perpendicular orientation at low pH and low salt concentration, and the fast aggregation regime of CNFs is realized by the reduction of electric repulsion for both perpendicularly and parallelly interacting CNFs. Meanwhile, the application of Smoluchowski's equation to the mobility of CNFs results in the underestimation of the zeta potential.