Smith, 78, who has won two Emmy Awards and a Golden Globe for her role as the acid-tongued Lady Grantham, tells CBS in an upcoming TV interview that she is amazed by the popularity of the show both in Britain and the United States.
Asked whether she is proud of "Downton Abbey", Smith said, "I haven't actually seen it. So I don't, I don't sit down and watch it," she told CBS reporter Steve Kroft in the interview to be broadcast on Sunday.
"Never?" Kroft asked. "No, I haven't watched it," Smith replied, according to an advance excerpt released on Thursday.
Smith, known as a perfectionist, said that watching herself would make her worry about her performance. "It's frustrating. I always see things that I would like to do differently and think, 'Oh, why in the name of God did I do that?' "
"Downton Abbey," a drama about the lives of aristocratic Britons and their servants in the early 20th century, attracted some 7.9 million viewers for its third season premiere in January on U.S. television.
Smith's snobbish Lady Grantham and her withering remarks like "No-one wants to kiss a girl in black," and "What is a weekend?", have made her the show's biggest star. Smith also has two Oscars for her roles in 1969 film "The Prime of Miss Jean Brodie" and 1978 comedy "California Suite" but she said she had no interest in her recent fame.
"I don't feel any different to the way I felt before and I'm not quite sure what (being a star) means. I am familiar to people now, which is what I was not before," she told Kroft. "That is entirely due to the television set."
Smith also acknowledged her reputation as an actress who doesn't suffer fools gladly. "Old people are scary and I have to face it, I am old and I am scary and I am very sorry about it, but I don't know what you do," she quipped.
The full interview can be seen on "60 Minutes" on February 17 on CBS television.