The Oscar ceremony turned solemn as Ledger received a standing ovation from his Hollywood colleagues for his turn in the latest Batman saga.
Ledger died in January 2008 of an accidental overdose of prescription drugs at age 28. His parents and sister spoke on his behalf Sunday.
"This award tonight would have humbly validated Heath's quiet determination to be truly accepted by all you here tonight, his peers within an industry he so loved," said his father, Kim Ledger.
The award will go to "your beautiful Matilda," said sister Kate Ledger.
"You just have to look at Matilda to know she's totally like her daddy. She has the same mannerisms, and I really feel he's in her," Sally Bell, Heath's mother, said backstage.
Kate Ledger recounted how her brother sent her "bits and pieces" of the unfinished "Dark Knight, and her prediction when he came home to Australia for a Christmas 2007 visit.
"I said, `I have a feeling, this is it for you ... You're going to get a nomination from the Academy,' Kate Ledger said. "He just looked at me and smiled."
The Australian actor recreated the Batman nemesis as a new-age villain, complete with ironic tales of emotional woe and garish makeup.
Ledger became the second posthumous acting winner in Academy Award history. Peter Finch won the first for 1976's "Network."
Other nominees in the category were Josh Brolin for "Milk"; Robert Downey Jr., "Tropic Thunder"; Philip Seymour Hoffman, "Doubt" and Michael Shannon, "Revolutionary Road."
Ledger's performance already had received Golden Globe and Screen Actor Guild awards and was the clear favorite for the Oscar. He was nominated previously for his role as a ranch hand involved in a secret gay relationship in 2005's "Brokeback Mountain."
Ledger's Oscar nomination for "Dark Knight" came exactly one year after his death.
He has one movie yet to be released. He died in the midst of production on "The Imaginarium of Dr. Parnassus," and director Terry Gilliam has salvaged the unfinished performance by enlisting Jude Law, Johnny Depp and Colin Farrell to complete the part.
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