The 37-year-old, who won an Oscar for "La Vie En Rose", said that that she studied the English language more seriously after landing Hollywood roles, reported Digital Spy online.
"Well, first of all, I love the English language, so it makes it easier to learn. And also when I was in Los Angeles in 2008, and I met with Michael Mann," Cotillard said.
"We did this movie together, I really had to work on my American accent. And I worked for four months, every day, with a dialect coach, and I improved my English a lot.
"I still have a lot of work, but I feel much more comfortable than a few years ago when I got here and I couldn't express myself properly," she added.
Cotillard has landed a variety of high-profile roles in English-language films in recent years, including "9", "Inception" and "The Dark Knight Rises".
Her current French-language romance film "Rust and Bone" is now playing in the US and the UK.