uses
  ComObj; 

// Replace Flags 
type 
  TWordReplaceFlags = set of (wrfReplaceAll, wrfMatchCase,
  wrfMatchWildcards);

function Word_StringReplace(ADocument: TFileName; SearchString,
ReplaceString: string; Flags: TWordReplaceFlags): Boolean;
const 
  wdFindContinue = 1; 
  wdReplaceOne = 1; 
  wdReplaceAll = 2; 
  wdDoNotSaveChanges = 0; 
var 
  WordApp: OLEVariant; 
begin 
  Result := False; 

  { Check if file exists } 
  if not FileExists(ADocument) then 
  begin 
    ShowMessage('Specified Document not found.'); 
    Exit; 
  end; 

  { Create the OLE Object } 
  try 
    WordApp := CreateOLEObject('Word.Application'); 
  except 
    on E: Exception do 
    begin 
      E.Message := 'Word is not available.'; 
      raise; 
    end; 
  end; 

  try 
    { Hide Word } 
    WordApp.Visible := False; 
    { Open the document } 
    WordApp.Documents.Open(ADocument); 
    { Initialize parameters} 
    WordApp.Selection.Find.ClearFormatting; 
    WordApp.Selection.Find.Text := SearchString; 
    WordApp.Selection.Find.Replacement.Text := ReplaceString; 
    WordApp.Selection.Find.Forward := True; 
    WordApp.Selection.Find.Wrap := wdFindContinue; 
    WordApp.Selection.Find.Format := False; 
    WordApp.Selection.Find.MatchCase := wrfMatchCase in Flags; 
    WordApp.Selection.Find.MatchWholeWord := False; 
    WordApp.Selection.Find.MatchWildcards := wrfMatchWildcards in Flags; 
    WordApp.Selection.Find.MatchSoundsLike := False; 
    WordApp.Selection.Find.MatchAllWordForms := False; 
    { Perform the search} 
    if wrfReplaceAll in Flags then 
      WordApp.Selection.Find.Execute(Replace := wdReplaceAll) 
    else 
      WordApp.Selection.Find.Execute(Replace := wdReplaceOne); 
    { Save word } 
    WordApp.ActiveDocument.SaveAs(ADocument); 
    { Assume that successful } 
    Result := True; 
    { Close the document } 
    WordApp.ActiveDocument.Close(wdDoNotSaveChanges); 
  finally 
    { Quit Word } 
    WordApp.Quit; 
    WordApp := Unassigned; 
  end; 
end; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  Word_StringReplace('C:\Test.doc','Old String','New String',
  [wrfReplaceAll]); 
end;