SelAttributes.Style method of RichEditComponent. You may set bold, italic, underline, strikeout style and other parameters of font in RichEdit. And don't forget to return focus to RichEdit component.
procedure TForm1.Button1Click(Sender: TObject); begin if fsBold in RichEdit1.SelAttributes.Style then RichEdit1.SelAttributes.Style:= RichEdit1.SelAttributes.Style-[fsBold] else RichEdit1.SelAttributes.Style:= RichEdit1.SelAttributes.Style+[fsBold]; RichEdit1.SetFocus; end;