Reports Deadline, "Like it did with Boardwalk Empire in September, HBO just renewed Game of Thrones moments after the ratings for the premiere episode were in. For Game of Thrones, those ratings were solid, not great. But, also like with Boardwalk Empire, HBO invested in elaborate sets and designs in re-creating the world from George R.R. Martin's books, so a second season was considered a sure thing."
For more ratings details, follow the link to Deadline.