The film, which stars actress Naomi Watts, focuses on the romantic relationship between 'the People's Princess' and heart surgeon Dr Hasnat Khan. It also puts the humanitarian work she did to help the victims of landmines in the spotlight.
Diana will "introduce a time in the Princess's life that was uniquely important in shaping her final years, fulfilling her search for true happiness and sealing her legacy," according to the official synopsis.
The trailer includes the snippets of memorable scenes of Diana's life recreated by British-Australian actress Naomi, Naveen Andrews (Dr Khan) and Cas Anvar (Dodi Fayed).
In May, twice Oscar-nominated actress Naomi revealed that she was nervous about playing Diana.
"It was very scary going into it," said the 43-year-old Australian actress at Cannes. "It was interesting getting to know her as I did all the research.
"She is a fascinating woman who led an extraordinary but short life. The most fun part of it is the research. It was daunting because everyone feels they know her so well so you want to get it perfect. It's not an impression, it's an interpretation.
"It's also more complicated and sensitive when you take into account her two boys, William and Harry. I want to do her justice and help tell her story truthfully."
Naomi, who starred in 2012 film The Impossible, said, "It is such an honour to be able to play this iconic role.
"Princess Diana was loved across the world and I look forward to rising to the challenge of playing her on screen."
The movie is directed by Oliver Hirschbiegel, whose previous credits include Downfall, a film about the last five days of Adolf Hitler's life.
Oliver added: "I am delighted to have such a truly exceptional actress who embodies the warmth, humanity and empathy of such a global icon as Princess Diana."
The film, which was originally rumoured to be called Caught in Flight, is based on a screenplay by playwright Stephen Jeffreys, who previously penned the script for the 2004 Johnny Depp film The Libertine.
Diana will have its world premiere in London on 5 September before hitting cinema screens on 20 September.