Is this move allowed ?

Watch this game , Move number 27 , black take white pawn by crosdsing it, Is this move allowed ?

Yes, it's called En Passant. Look it up, it's an interesting rule.

I presume move 27 was a2-a4 and Black then played bxa. Yes it's legal, it's called en passant: http://en.wikipedia.org/wiki/En_passant

If it happened to you don't feel bad. It can be a shocking move to new players that don't know about it, and has led to countless arguments when the "victim" accuses the person who used En Passant of cheating. Just remember it for future games. 

Enpassant rule. Every chess lover must know.

It's legal by the very fact that the move was allowed by the computer, e.g., it wont allow you to move into check or castle across a check.

How is it that you made the move as black but asked if it was legal in this thread?

Em passant: French for "a passing win"

French for "In passing" I think you mean.

Fastest "book" en passant that makes some sense- factly, this used to be a Petroff mainline some thirty years ago.