Many families would not be able to buy the massive book if it was sold as one novel.
Parts of the fifth Harry Potter book will be published every week and will cost kids about 30 cents a time.
'All my friends are keen'
OOTP has been flying off the shelves in Vietnam since it was released on 28 July. It took longer to be released there because it had to be translated into Vietnamese first.
Staff at the company which publishes Potter in Vietnam, The Tre Youth Publishing House, said they would be printing about 40,000 copies of OOTP - about 10,000 more than any of the earlier books.
The person who translated the Potter books into Vietnamese, Ly Lan, said it was a struggle to get them published there at first because foreign books don't usually sell well.
But the magic of Harry won them over in Vietnam too.