Postdoc without letter of recommendation What if my PhD advisor doesn't want to give me a letter of recommendation, because he doesn't want me to leave on postdoc? No one of his young assistants have never been on any postdoc or fellowship exchange.
Based on your comment responses, I think that you are working for a young faculty member who is trying to build his group and encouraged his first two PhD students to stay on after they graduated. It's an admirable goal to start building a research group, but in the US it would be discouraged to do so by keeping all of your own students in my experience. US academic research has a bias towards sending one's students out into the world to cross-pollinate ideas and avoid stagnation and closed-mindedness.

That being said, refusing to provide a letter of recommendation to a good student for this purpose is borderline if not outright professional misconduct and wouldn't be tolerated at most US universities (though I'm constantly surprised what weak administrators let star faculty get away with sometimes). 

I think you should sit down with your advisor and explain your life goals to him. Include the fact that you are eager to get out into the world and learn more in a different environment. Try to convince him to see it from your perspective without getting antagonistic. If that doesn't work, go see your department chair, but only as a last resort. 

You are in a bit of a tenuous position since your advisor could refuse to sign off on your dissertation (or let you defend, or drag his feet on revisions, etc) if he becomes scared by feelings of abandonment in your desire to leave. Bringing in higher authorities could make this worse. It would be best to convince him with the power of good arguments than to make the situation adversarial. 

The timing of this can be tricky. He's probably going to find out that you applied for postdocs with out his recommendation and blessing, and given the usual schedule for postdoc recruiting, I'm assuming you will have to apply before you defend. I think it's probably better to be open with him that you are definitely going to apply, find a postdoc, and leave than to do so behind his back. Maybe when he sees your resolve to do so, he will change his mind about writing you a good letter.