The work aims to prepare cellulose Ⅱ nanofibers with high aspect ratio, and compare their properties with those of cellulose Ⅰ, Ⅰ/Ⅱ nanofibers. Cellulose Ⅰ, Ⅰ/Ⅱ and Ⅱ nanofibers (CNF-Ⅰ, CNF-Ⅰ/Ⅱ and CNF-Ⅱ, respectively) were prepared by chemical treatment and mechanical grinding with corn straw as the raw material. Transmission electron microscopy showed that the diameter distribution range of CNF-Ⅰ, CNF-Ⅰ/Ⅱ and CNF-Ⅱ were about 5~50 nm, 5~60 nm and 5~80 nm, respectively. Ultraviolet-visible spectrophotometer test exhibited that the three CNF films all had good transmittance. Mechanical tests showed that CNF-Ⅰ film had higher the tensile strength (184.7 MPa), while CNF-Ⅱ film was more ductile and had higher elongation at break (12.5%). During preparation of CNF-Ⅱ, the delignification technology has a great influence on the crystal transformation (from cellulose Ⅰ to cellulose Ⅱ) and subsequent mechanical nano-fibrillation. CNF-Ⅱ with high aspect ratio can be prepared by chemical treatment combined with mechanical grinding method. The yield rate of once grinding is up to 80%. It has the advantages of simple process, easy operation and low production cost.