"I neither will aspire to nor will I accept, the position of president of the Council of state and commander in chief," he wrote in the letter.
Mr Castro handed over power temporarily to his brother, Raul, in July 2006 when he underwent intestinal surgery.
The 81-year-old has ruled Cuba since leading a communist revolution in 1959.
In December, Mr Castro indicated that he could possibly step down in favour of a younger generation.
The National Assembly is scheduled to meet on 24 February to elect his successor.
Lets hope that now the Cuban people can get back to real life ...
