"I knew I was in the running, yes, but I never thought I would win," Munro said, in comments reported by the CBC.
The writer said that she had been awakened by her daughter with the news that Sweden's Nobel Committee had given her its literature prize.
She said she always viewed her chances of winning the Nobel as "one of those pipe dreams" that "might happen, but it probably wouldn't".
"It's the middle of the night here and I had forgotten about it all, of course," she told the Canadian broadcaster.
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Munro won the Nobel Literature Prize for her short stories that focus on the frailties of the human condition, becoming just the 13th woman to win the coveted award, and the first Canadian.
The Swedish Academy described Munro, 82, as a "master of the contemporary short story."
The writer told the CBC that her husband, who died a few months ago, would have been very happy, but said she likely would not reconsider an earlier vow to abandon writing "because I am getting rather old.