The letter, written hours before Diana began a summer break during which she and Dodi Fayed died in a car crash in Paris, is dated August 11, 1997, and is written on Kensington Palace headed paper.
The letter, signed off "With love from Diana", was penned after a trip to Bosnia where she met survivors of landmine injuries as part of her charity work which was of vital importance to her.
In the letter, Diana says that her trip to Bosnia had "strengthened her resolve" to spread the message about landmine victims.
"I could not help but be deeply moved by the experience which hardened my resolve to ensure that the world does not forget that those who have been so needlessly maimed by these terrible weapons will need care and support for many years to come," Diana says in the letter.
Fraser, who knew Diana personally, sold the letter in 2007 during the 10th anniversary of her death and it was bought by a private British collector.
It is expected to fetch 3,000 pounds when it is sold at auction on April 28.