از این تابع برای تغییر شکل پنل خود به پنل با گوشهای خمیده استفاده کنید

procedure TForm1.FormCreate(Sender: T);
const
  bgcolor = $00FFDDEE;
  linecolor = $00554366;
var
  img: array of TImage;
  reg: hrgn;
  i: Integer;
begin
  for i := 0 to ComponentCount - 1 do
  begin
    if Components[i].ClassName = "TPanel" then
    begin
      setlength(img, Length(img) + 1);
      img[i] := TImage.Create(Self);
      img[i].Width := (Components[i] as TPanel).Width;
      img[i].Height := (Components[i] as TPanel).Height;
      img[i].Parent := (Components[i] as TPanel);
      img[i].Canvas.Brush.Color := bgcolor;
      img[i].Canvas.pen.Color := bgcolor;
      img[i].Canvas.Rectangle(0,0,img[i].Width, img[i].Height);
      img[i].Canvas.pen.Color := linecolor;
      img[i].Canvas.RoundRect(0,0,img[i].Width - 1,img[i].Height - 1,20,20);
      reg := CreateRoundRectRgn(0,0,(Components[i] as TPanel).Width,
        (Components[i] as TPanel).Height, 20,20);
      setwindowrgn((Components[i] as TPanel).Handle, reg, True);
      delete(reg);
    end;
  end;
end;

برگرفته شده از سایت دنیای برنامه نویسی دلفی(http://mt85.blogfa.com)