"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, who accepted the award with his family.
"Tonight, we are choosing to celebrate and be happy for what he has achieved," Sally Bell, Heath's mother, said of her son.
Sister Kate Ledger said the family was accepting the award on behalf of "your beautiful Matilda," Ledger's 3-year-old daughter with actress Michelle Williams.
Ledger, an Australian. 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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