Potter fans will be able to get their hands on copies of Harry Potter and the Half-Blood Prince on 16 July 2005.
Like the previous three Potter books it will be published on the same day in Britain and America.
According to British publishers Bloomsbury, the book "is definitely shorter" than Order of the Phoenix.
Some book shops may open at midnight, which means that UK fans will get the book a few hours earlier than American ones, because of the time difference.
The fifth book cost £16.99, while the fourth one cost £14.99.
The Half-Blood Prince is priced at £16.99, indicating that it might be close to the length of the previous two books, but perhaps closer to the length of the Order of the Phoenix.
Similarly, in America, the book will cost $29.99, the same as the fifth book.
A spokesperson for Scholastic, JK's American publishers told CBBC Newsround Online that the book was still being edited, and that its length would be announced sometime in the future.
JK announced she had finished writing on her website
On Monday JK Rowling used her website to reveal to fans that she had finished writing the sixth book in the series.
JK added that it had been handed over to her publishers Bloomsbury.
The author has already revealed a few details about the book.
Answering queries from fans she said that the Half-Blood Prince was neither Harry nor Voldemort.
She let us know three of the chapter titles:
- Chapter two - Spinners End
- Chapter six - Draco's Detour
- Chapter 14 - Felix Felicis
And there'll be a new Minister for Magic.
She even gave fans an excerpt: "He looked rather like an old lion. There were streaks of grey in his mane of tawny hair and his bushy eyebrows; he had keen yellowish eyes behind a pair of wire-rimmed spectacles and a certain rangy, loping grace even though he walked with a slight limp."
There's one more Harry Potter book to go after Half-Blood Prince.
JK Rowling has already written the last chapter of the seventh book, where she describes what happens to the characters who survive until the end.