We report the successful use of a high-speed blender in nanofibrillating never-dried pulp to cellulose nanofibers (CNFs) with a uniform diameter of 15-20 nm. Pulp treated for 30 min in a blender showed the same degree of fibrillation with less damage to the CNF compared with that treated in a grinder. Observing the process of nanofibrillation clarified that the straw-like pulp was fibrillated in a very characteristic way, by forming many "balloon-like structures". As the balloons extended to the edges, the fibrils were rapidly individualized. However, the pulp fragments with ripped cell walls were split into finer fragments and gradually disintegrated into nanofibers. Changing the agitation speed and pulp concentration during the treatment revealed that the pulp concentration of 0.7 wt % at 37,000 rpm was the optimum fibrillation condition in this blender method. Through treatments in various NaCl solutions, the effect of the surface charge of CNF on the fibrillation was studied from the viewpoint of colloidal surface physics. It was found that repulsion due to the electric double layer of the CNF surface may play a critical role in the occurrence of fibrillation.