John Nott wrote the private note when he resigned as defence minister from Thatcher`s Conservative government in 1983, and it has now been made public.
"Your greatest triumph as a PM, if I may say so, is that your colleagues actually like you. Some of them even love you, just a little!" he wrote to Thatcher, who died in April this year, aged 87.
Nott continued: "It is inexcusable to say so nowadays but I actually admire you as a woman -- your good looks, charm and bearing have always attracted me, as a man.
"I`m sorry, but what is wrong with that!"
"I think your emotional, instinctive and unpragmatic approach to most issues -- so very unmasculine -- is the secret of your success in the male-dominated world of politics."
Nott referred to the letter, which he signed "Love - John", in his memoir and revealed that Thatcher did not reply.
Thatcher was prime minister between 1979 and 1990. Her private papers from 1983 have been released by the Margaret Thatcher Foundation.
Nott, 81, is best remembered for being Britain`s defence minister during the 1982 Falklands War.