A shapefile's FID and Shape field and a dBASE table's OID field can't be deleted. The OID field is a virtual column created by ArcGIS software when accessing the table's contents; it guarantees that each record in the table has at least one unique value. Shapefiles and dBASE tables must have at least one attribute field in addition to the FID and Shape fields or the OID field.
You can also delete fields with the Delete Field tool.
- In ArcMap, right-click the shapefile layer in the table of contents and click Open Attribute Table.
- Right-click the field heading in the table and click Delete Field.
- Click Yes in the warning window.