property Underline: LongWord;

Underline can be one of these XlUnderlineStyle constants:

xlUnderlineStyleNone

xlUnderlineStyleSingle

xlUnderlineStyleDouble

xlUnderlineStyleSingleAccounting

xlUnderlineStyleDoubleAccounting

This example sets the font in the cell A1 to single underline.

Workbook.Worksheets[1].Range['A1', 'A1'].Font.Underline := xlUnderlineStyleSingle;