The princess was 36 when she died after crashing in the Pont de l'Alma tunnel on August 31, 1997.
The crash also killed her partner Dodi Fayed and the driver, Henri Paul.
Her 61-year-old brother took to Instagram on Sunday, sharing a photo of a bunch of white and pink flowers that he placed on Princess Diana's burial site on an island in the middle of an oval lake at his Althorp Estate in the English countryside.
"Flowers we cut this morning from Althorp gardens for the island," Earl Spencer captioned the photo.
"Always an impossible day."
The news of Princess Diana's death rocked the world, and millions of people continue to share tributes and pay their respects to the beloved princess.
Diana - who was married to King Charles, 76, from 1981, split from him in 1992, and their divorce was finalised in 1996.
Her sons William, Prince of Wales, then 15, and Prince Harry, who was 12 at the time, were on holiday with their father, then-Prince Charles, at Balmoral Castle in Scotland.
She was originally meant to be buried in the Spencer family vault at nearby St Mary's Church in Great Brington.
However, her brother decided to change the plan and have her laid to rest on the small, tree-covered island in the centre of the oval lake.
He admitted after Diana's death that he was haunted by the feeling he could have done more to save her from her tragic end.
"You always think, God, I wish I could've protected her. It was just ... it was devastating," he told People in 2017
"I always felt intensely protective towards her."
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