The Queen, Prince Philip and Prince Charles, as well as friends of the royal family, joined Princes William and Harry for the service, which was held at Guards' Chapel near Buckingham Palace.
Prince Harry was 12 when his mother Diana was killed and he said her death changed his life and that of his brother William forever.
Prince William paid tribute by giving a reading from St Paul's letter to the Ephesians and Princess Diana's sister, Lady Sarah McCorquodale, also gave a reading.
UK Prime Minister Gordon Brown and his predecessors Tony Blair and John Major were also at the service accompanied by their wives.
Additional memorial services to mark the Princess of Wales’ death are being held around the UK.
