The actress battled with nerves at the thought of stripping off and locking lips with Kunis in Darren Aronofsky's forthcoming film.
Portman told Showbiz Spy: “I was terrified. At the moment I lived in a state of inner terror.”
Natalie stars as a veteran ballerina in New York who finds herself in competition with a mysterious rival dancer (Kunis).
The actress, who found love on set with French dancer Benjamin Millepied, is worried that the steamy girl-on-girl action will distract viewers from the film's story.
“Nudity is something absolutely natural and I’m not prudish — but in a film it can be distracting. Doing Black Swan I couldn’t say no."
"The project is a huge opportunity for me to show a new me. I can’t always play the nice mothers. My other problem is that single pictures are being taken out of context and put on the internet."
The 28-year-old previously shot a nude scene for short film Hotel Chevalier directed by Wes Anderson, which was a prelude to the movie Darjeeling Ltd.
Vincent Cassel and Winona Ryder also star in the Black Swan which is Aronofsky's follow-up to The Wrestler.