I'm in a long distance relationship... we've met about an year ago and it was indeed the happiest month of our lives. I'm about to move in with him but he's so jealous! It's getting worse with time. He even threaten me... I'm not going to cheat on him and he said that it will be good when I get here. What could I do to assure him that I'm faithful?

You cant.

Either he trusts you now or he dos'ent, moving in with him won't change that, in fact chances are he will become worse.

The first month was great, it usually is, people put there best side out your all loved up, time passes and the REAL person emerges and that's the guy your seeing now.

He has threatened you! This guy has issues that are not yours to deal with, don't put yourself in a situation where you are cut off from family and friends and end up relying on him..

My advice, don't move in, break it off and run like hell in a different direction.

A relationship is built on trust and if he can't trust you when you're not with him all the time then maybe you need to rethink a few things.
Sure, you're moving in with him, which will probably ease his concerns to begin with. But then what happens when you leave the house for work? Or go out with friends? If he's already suspicious and accusing you now, then it won't stop just because you're moving in with him, it's a trust issue that he has and that he needs to work through, whether he's with you or not.

When relationships are filled with doubt and distrust, they can often become stifling, and you will end up being pushed apart from each other. The best thing you can do is to sit him down and tell him that you are not the cheating type, and you have absolutely no intention of straying away because you love him. But at the same time, you need to make it known that you will not be in a relationship where you are constantly questionned and interrogated when you are completely innocent.

If he can't handle this and continues, then it's not fair for the relationship to continue. But you shouldn't have to stop doing the things you want to do, such as going to work or going out with friends, just so he can keep his eye on you at all times. That's not a relationship, it's like living in a prison.

This isn't you're problem you need to solve, this is his issue with trust. You have been and always intend to be faithful to this guy, and have never given any indication that you want to cheat on him.
Sure, for the first week or so, living together will be great, but what do you think he will be like when you go to work? Or have to go out to do the shopping, or meet up with friends? He's still going to be jealous and paranoid that you're cheating and the questionning will never stop.

Don't even contemplate moving in with him until he's acknowledged he has a problem and tries to address it. In the meantime, let him know that you are there for him to support him, and you will not cheat on him.