Dear Halildurak, 
 
Two standard reference works in the UK give �as if � and �as though � together, with the same meaning.
 
Oxford Dictionary of English, 2006: "as would be the case if": �she behaved as if he wasn �t there �.
 
The Chambers Dictionary, 2003: "as it would be if".
 
There is no difference between the two!
 
However, be careful!
 
The expression: "As if!", standing alone, can NOT be interchanged with "As though!", because, "As though!" does not exist in English!
 
ODOE: "As if!" [informal] I very much doubt it.  "You know how pools winners always say it won �t change thir lives?  Yeah, as if!"
 
TCD: "As if!" interjection, expressing incredulity.
 
I hope that this helps you.
 
Les