Can i marry my cousin's daughter?

You want to marry your first cousin - once removed. In the US, there are four states where that is banned. Check out the table in this article.

what about outside US?
what catholic church says?
and what is the real reason for banning this?

It seems that the Canon Law doesn't oppose that kind of marriage.

But check out these links attentively:

Cousin

Cousin marriage

This is a pretty good explanation re the Catholic Church's position - http://catholicexchange.com/can-cousins-marry-in-the-church/

According to that site, the relationship is, I believe, the fifth or the sixth degree, and is therefore permitted.

HOWEVER,

the Catholic Church needs to abide by local laws. "Outside of the US" is extremely vague and involves over 100 countries. You will have to be considerably more specific than that, and we are not going to go hunting in every single country's law books to try to find one where everything is hunky-dory.

Per the above-mentioned site, the reasoning is based upon "natural law", but recognize that that is considered to be ambiguous the United States (see: http://en.wikipedia.org/wiki/Natural_law#American_jurisprudence) and is likely considered ambiguous elsewhere in the world. Certainly if a written law is on the books somewhere, and it supercedes alleged "natural law", it is likely to be the written document that will prevail (not to say you can't argue it - argue whatever you please - but your chances of winning against a well-documented, well-argued bit of legislation by citing vague "natural law" - those odds aren't so red-hot).

Ok.. Thanx everyone.
Also what science says about this kind of relationship?

so what happen