He added: 'She was one of the naughtiest parents. Read on to see some of their best moments together.
Adding that despite her royal title, she was a hands-on and cheeky mum.
The full documentary will be shown on Britain's ITV channel later this month.
"Believe it or not, you and I are both in this photograph, you're in the tummy", William tells his little brother.
Princess Diana's tragic death on August 31, 1997 is being marked by a slew of TV specials. "She smothered us with love, that's for sure", Harry reflected.
Diana, by her sons
People around the world were devastated when Diana died in a vehicle accident the following year in Paris.
Harry spoke of becoming "a problem" to himself, but "did not know what was wrong".
The brothers, who both opened up publicly this year about the grief they felt after their mother's death, gathered with family members last Saturday, on Diana's birthday, at her family home, Althorp House, to rededicate her grave in a private service. Her death gripped the nation and highlighted the often intrusive nature of paparazzi, whom many blamed for her death as it was widely reported that the vehicle Diana had been in was speeding to escape photographers.
The vehicle was being driven by the pair's drunk chauffeur Henri Paul, who attempted to outrun the pursuing paparazzi.
Diana's death would send shockwaves around the world. The siblings will also "offer their perspective on the Princess' unique contribution to public life, including her charitable and humanitarian work", according to Oxford Film and Television, which is producing the film.
But for her two sons, Priness Diana will always be a mischievous mother, whose love of life and fun was infectious.More news: At G20 summit, Trump promises $639 million in food, humanitarian aid