The reality TV mogul claims he was "like a schoolboy" for Dannii who he started sleeping with after she joined the panel The X Factor in the UK alongside him in 2007.
In a new unauthorised biography, Sweet Revenge: The Intimate Life of Simon Cowell, he says: "I had a crush on her. It was Dannii's hair, the sexy clothes, and the tits.
"I was like a schoolboy. She was foxy. She was a real man's girl. Very feminine. It was genuine love."
Simon, 52, started seeing Dannii - who earlier this month split from former rugby player Kris Smith after four years together - after breaking up from his long term girlfriend Terri Seymour, in 2008.
He added: "After Terri Seymour, I wasn't ready for another relationship."
Dannii - who is now a single mother to 21-month-old son Ethan - left The X Factor in 2010, and by that time Simon already had his eye on another of the show's female judges, pop singer Cheryl Cole, who was on the judging panel between 2008 and 2010, although he was unsuccessful in getting her.
'She played me'
He said: "I would have liked an affair with Cheryl [but] I felt like a mouse being played with by a beautiful cat."
Simon was smitten with the 28-year-old star after she made frequent visits to his dressing room while they starred on the show.
In quotes from the book published in The Sun newspaper, Simon said: "She would come in dressed in her tracksuit and her slippers, drop her eyes and play the soulful victim to get around me. She played me. I adored her."
However, Simon and Cheryl's relationship ultimately turned sour after he took her to star in the US version of The X Factor but she wasn't a hit, and was sacked after just three audition shows.
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