Thank you for your replay.
I tried your code but its not working.
this is my code:
var sLookupValue = new Array();
sLookupValue = Xrm.Page.getAttribute("sislookupfield").getValue();
if(sLookupValue [0] != null)
{
var sisid= sLookupValue [0].id;
}
var siscodedesc = function(sisid);
Xrm.Page.getAttribute("sicdescription").setValue((siscodedesc));
}