Heath Ledger 
The role of the Joker was played by Heath Ledger in the Dark  Knight. Sadly he passed away not long after this movie was done but  he did win an Oscar posthumously for this … role. Heath Ledger kept a chilling diary in preparation for his Oscar-winning portrayal of the Joker in The Dark Knight. The Australian actor died of an accidental overdose of a combination of prescription medications in January 2008. Remember when Heath Ledger was first cast as Batman's arch-nemesis the Joker for Christopher Nolan's 2008 The Dark Knight sequel? Warner Bros. The late Heath Ledger played the Joker in The Dark Knight. Ledger died on 22 January 2008 from an accidental intoxication from prescription drugs. A few months before his death, Ledger had finished filming his performance as the Joker in The Dark Knight. His death occurred during editing of The Dark Knight and in the midst of filming his last role as Tony in The Imaginarium of Doctor Parnassus. Before this shooting spree, one of the greatest tragedies associated with the film was the death of Heath Ledger, who played the Joker in The Dark Knight. One of the greatest tragedies to strike previously was the death of Heath Ledger, who played the Joker in The Dark Knight. The actor was found dead in his Manhattan apartment in January 2008. Heath Ledger played the Joker in the Dark Knight. Heath Andrew Ledger (4 April 1979 – 22 January 2008) was an Australian actor and director. After performing roles in Australian television and film during the 1990s, Ledger left for the United States in 1998 to develop his film career.
http://www.answers.com/Q/Who_is_the_actor_that_played_the_Joker_in_The_Dark_Knight http://www.independent.co.uk/arts-entertainment/films/news/heath-ledgers-father-reveals-dead-actors-joker-diary-written-during-the-dark-knight-8640660.html http://www.businessinsider.com/heath-ledger-was-criticized-before-played-joker-2013-8 http://www.answers.com/Q/Who_is_the_actor_that_played_the_Joker_in_The_Dark_Knight https://en.wikipedia.org/wiki/Heath_Ledger http://www.dailymail.co.uk/news/article-2176696/The-Dark-Knight-Heath-Ledgers-death-Morgan-Freemans-accident--How-Batman-franchise-plagued-tragedy.html http://www.answers.com/Q/Who_is_the_actor_that_played_the_Joker_in_The_Dark_Knight https://en.wikipedia.org/wiki/Heath_Ledger
actor that played joker in the dark knight