The fabrication of polysaccharides to be nanoparticles or nanocapsules is quite specific due to various parameters and factors. The present work demonstrates a simple pathway to selectively prepare the ionic polysaccharide flakes to be nanoparticles or nanocapsules. The systematic studies on the model cases of cationic polysaccharide (i.e., chitosan) and anionic polysaccharide (i.e., alginate) confirm that pK(a) is the key point to tune the polysaccharides to be nanoparticles or nanocapsules. When the ionic polysaccharides were in an oil/water emulsion system, the pH close to pK(a) leads to the densely packed polysaccharide chains under the hydrogen bond networks, and as a result the cross-link occurs all through the chains to be nanoparticles. On the other hand, when pH was adjusted to the lower or higher than pK(a) depending on the types of ionic polysaccharide, the polysaccharide chains are under charge-charge repulsive force, resulting in the alignment of polysaccharide chains to be hollow nanospheres, and at that time the cross-link initiates the formation of nanocapsules. The present work, for the first time, clarifies that pH variation is the key to selectively prepare nanoparticles or nanocapsules, and this is important for delivery systems, coatings, sensors, and so forth.