"She was one of the most fun people," he penned.
"She was incredibly indiscreet, you could ask her anything and she would tell you," he added.
The singer, who sang Candle In The Wind at her funeral, confessed that her playful manner changed when she spoke of her husband Prince Charles.
"She called him her husband, never Charles. It seemed very cold and formal, which is strange because the one thing Diana wasn’t as formal," he recalled.
After her separation from Charles, according to Elton, Diana enjoyed her single life.
"Men seemed to lose their mind in her presence," he said.
When Princess Diana died tragically in 1997, Elton was devastated.
He reflected, "It was one of the greatest friendships of my life."
Elton John has continued his love for Diana, through protecting her youngest son Prince Harry - and his wife Meghan Markle.
The legendary singer - who performed at the couple's royal wedding last year, came out in defence of the pair.
"Prince Harry’s Mother, Diana Princess Of Wales was one of my dearest friends. I feel a profound sense of obligation to protect Harry and his family from the unnecessary press intrusion that contributed to Diana’s untimely death."