[O] src blocks not fontified

[Shiyuan]

Hi,
    I am using Emacs 24.3 with the built-in org mode(7.9.3). The src code
block is not fontified (Emacs is started with -Q option)
#+BEGIN_SRC emacs-lisp
    (defun org-xor (a b)
      "Exclusive or."
    (if a (not b) b))
#+END_SRC

This is my first time to use org-mode, so I could miss some setup steps.Do
I need to do any setup in my .emacs if I just use the built-in org?

Thanks.

Shiyuan