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