Pure cellulose have been isolated from rice straw at 36% yield and hydrolyzed (64% HSO, 8.75mL/g, 45°C) for 30 and 45min to cellulose nanocrystals (CNCs), i.e., CNC30 and CNC45, respectively. CNC45 was smaller (11.2nm wide, 5.06nm thick and 117nm long) than CNC30 (30.7nm wide, 5.95nm thick and 270nm long). Freeze-drying of diluted CNC suspensions showed both assembled into long fibrous structures: ultra-fine fibers (∼400nm wide) from CNC45 and 1-2μm wide broad ribbons interspersed with CNC clusters from CNC30. The self-assembled fibers from CNC30 and CNC45 were more highly crystalline (86.0% and 91.2%, respectively) and contained larger crystallites (7.36nm and 8.33nm, respectively) than rice straw cellulose (61.8%, 4.42nm). These self-assembled fibers had essentially nonporous or macroporous structures with the CNCs well aligned along the fiber axis. Furthermore, the self-assembled ultra-fine fibers showed extraordinary structural stability, withstanding vigorous shaking and prolong stirring in water.