The 42-year-old actress said she was told to "work out" for her role in the 2001 film, reported ET online.
"I don't think I fitted the type of actress Michael Bay, the director, had met before.
"I'd just had my daughter and had lost weight but was told that if I got the part I'd have to work out, and I just didn't understand why a 1940s nurse would do that," Beckinsale said.
"Pearl Harbor," which also starred Ben Affleck and Josh Hartnett, was Beckinsale's second role in a major Hollywood film following 1999's "Brokedown Palace".