Bush said in an interview with Florida newspapers posted on the website of the "Saint Petersburg Times" that the letter failed to address international demands that Iran stop work which could be used to make nuclear arms.
In Bush's words, the letter did not answer the question, "When will you get rid of your nuclear program?"
The letter's existence emerged on May 8 in what is believed to be the first presidential contact between the two countries in 27 years.
The letter focused on the failure of democratic governments and criticized Bush's foreign policy.
(AP, dpa, Reuters)