Daniel Radcliffe, Emma Watson, Rupert Grint and other cast members are expected at the screening in London's Leicester Square on Tuesday.
Grint said he would attend after recovering from a mild bout of swine flu. He said the illness was not severe and "just like any other flu I've had before."
The cast members plan to wear white ribbons in memory of Rob Knox, a young actor on the film who was stabbed to death in London last year.
The sixth Potter film sees the boy wizard and his friends battling evil Lord Voldemort - and facing raging teenage hormones.
The movie opens around the world July 15.
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