Royal fans flooded the past with kind messages of condolence and reminisced about Diana's lasting legacy.
"Thank you for posting this photo. Diana was truly The Queen of Hearts. Blessings to you and yours on this sad anniversary," one posted.
“The world has never been the same since that awful day. It makes me so sad that Diana lost her life so young. She truly is 'irreplaceable and whose beauty, both internal and external, will never be extinguished from our minds,'” another wrote, referring to a quote from Charles' eulogy that he read at Diana's 1997 funeral.
Tessy Ojo, CEO of The Diana Award that was established in the late royal's honour also replied: "She certainly was one of a kind. We are honoured to continue her legacy through the @DianaAward."
The Earl of Spencer also shared a photo of the many floral arrangements that had been left at Althorp House.
Though Diana's grave is located on an island in the lake at Althorp known as The Round Oval that is closed to the public, there is a lakeside temple where members of the public can pay their respects.
"Lovely flowers from all sorts of kind people today, and recently, at @AlthorpHouse: thanks to all of them," Charles captioned the photo.
Charles Spencer famously criticised the media for their treatment of his sister and took a subtle swipe at the royal family, calling her "the most hunted person of the modern age."
"She would want us today to pledge ourselves to protecting her beloved boys William and Harry from a similar fate and I do this here, Diana, on your behalf. We will not allow them to suffer the anguish that used regularly to drive you to tearful despair," the Earl of Spencer said at the time.
"And beyond that, on behalf of your mother and sisters, I pledge that we, your blood family, will do all we can to continue the imaginative way in which you were steering these two exceptional young men so that their souls are not simply immersed by duty and tradition but can sing openly as you planned."
Charles also walked behind his sister's coffin with her ex father-in-law, Prince Philip, her ex-husband Prince Charles and her sons Prince William and Prince Harry, who were 15 and 12 years old at the time, respectively.
An estimated two and a half billion people tuned in to watch Diana's state funeral at Westminster Abbey.
However, her close family then attended a private burial on The Round Oval where she was laid to rest.
When one Twitter user asked if it was a rue that Diana asked for an apartment on the Spencer Estate and Charles denied her for fear of paparazzi invasion, the late Princess of Wales' brother denied the claims entirely.
"No, that’s not true. And I’ve won three libel cases against newspapers who said that it was," he remarked.