Language Center Program, Writing Tips
Volunteer State Community College | Division of Humanities
Underlining and italicizing titles are "equivalent" formatting options. In the past, most writers did not have the ability to create italicized type, so underlining was used as a substitute. Some instructors prefer italics, in part because underlining is used to indicate a hyperlink in Web documents. So be sure to ask your instructor what he/she prefers. Or if that is not possible, choose one formatting option and use it consistently.
FYI: For a list of what titles should be underlined or italicized, be sure and consult a grammar handbook.