A separator for a nonaqueous secondary battery includes a plurality of cellulose nanofibers and a hydroxyl group-masking component for masking hydroxyl groups on a surface of the cellulose nanofibers, wherein the cellulose nanofibers are cross-linked by the hydroxyl group-masking component to form a nonwoven fabric; as well as a nonaqueous electrolyte secondary battery including the separator, and a method of preparing the separator.