You will need to send a LB_SETTABSTOPS message to the Listbox. Note: The Listbox tabwidth property must be set to a number other than zero for the LB_SETTABSTOPS mesage to work. Example: procedure TForm .FormCreate(Sender: TObject); begin ListBox .TabWidth := ; ListBox .Items.Add('one'+#9+'one'); ListBox .Items.Add('two'+#9+'two'); ListBox .Items.Add('three'+#9+'three'); end; procedure TForm .Button Click(Sender: TObject); var DialogUnitsX : LongInt; PixelsX : LongInt; i : integer; TabArray : array[0..4] of integer; begin DialogUnitsX := LoWord(GetDialogBaseUnits); PixelsX := SpinEdit .Value; for i := to 5 do begin TabArray[i - ] := ((PixelsX * i ) * 4) div DialogUnitsX; end; if SendMessage(ListBox .Handle, LB_SETTABSTOPS, 5, LongInt(@TabArray)) = 0 then ShowMessage('Tabs Not Set'); Listbox .Refresh; end;