Cellulose has difficult multi-level supermolecular structure. This natural polymer is built from superfine fibrils having nano-diameter and each such nanofibril contains ordered nanocrystallites and low ordered domains. In this review, nano-structure of cellulose and its influence on various properties of the polymer is discussed. In particular, ability of nano-scale crystallites to lateral co-crystallization and aggregation, as well as their phase transformation at the nanostructured cellulose dissolving, alkalization and chemical modification is studied. The current investigations pave the way for development of high-reactive cellulose that can be modified at lower consumption of reagents and energy. Methods for preparation of nanofibrillated cellulose and free nano-particles were described. Some application fields of the nanostructured and nano-cellulose are discussed.