Thank you guys,
I tried this, i got it...
if (e.CommandName == "Edit")
{
ImageButton btnedit = (ImageButton)e.Item.FindControl("imgEdit");
string[] cmdArgs = e.CommandArgument.ToString().Split(new char[] { ',' });
String strJSFunctionCall = String.Format("GetAlt({0},{1},{2})", cmdArgs[0], "'"+cmdArgs[1]+"'", 0);
btnedit.Attributes.Add("onclick", strJSFunctionCall);
}
its started working when i placed quotes for string.
Thanks Once Again everyone.