Taken in 1997 of the late Princess of Wales, the images are going on show at the family home of the Spencer'ss and her final resting place.
Mario initially shot them for Vanity Fair but they turned out to be the last official portraits taken before her death.
Twenty years later, the images will be on display in their entirety in the exhibition rooms.
Earl Spencer said: “I have always felt that Mario Testino captured the essence of Diana quite brilliantly. These wonderful images made a huge impact on me when I saw them first, and revisiting them now, 20 years later, seems a fitting tribute to my beautiful, fabulous, sister.”
It can be seen every day from Sunday May 28 to Sunday June 4. The house will be open other dates of the year. Visit www.spencerofalthorp.com for more information.