Hi @Nam Gi VU:
In general, this is not really a WordPress question. I'll answer it because I tend to be lenient about how related questions need to be but there's a good change another moderator will close the question and if they do I'll agree.
Basically since WordPress.com does not provide this feature you need to use a 3rd party service that will read your RSS feed and send tweets to your Twitter account:
P.S. One more thing. I do not recommend auto-tweeting blog posts. Most Twitter users (myself included) will unfollow people who auto-tweet if they recognize them doing it. I want to hear from the person, not something they've set up to run automatically. FWIW.