Coming off a victory last week in a Korean LPGA event, Ryu had five birdies and two bogeys in relatively calm conditions on the difficult links course.
"This is my first time played in England, so I never played this type of golf course," Ryu said. "It's really tough, but fun. Always the first experience, really fun and a little tough, but I want to enjoy this type of golf course.
"Actually, Tuesday and Wednesday was so bad, so today feels like a really great weather. But you know, in Korea it was a little really strong wind, and a little different from this course. This wind might not be a bad wind. I think today the weather was really great."
The 2-under leading score is the highest in the first round since the tournament became a major in 2002, in relatively calm conditions.
The 21-year-old Kang, also from South Korea, had six birdies, two bogeys and a double bogey.
"I hit it pretty good out there," said Kang, winless on the LPGA Tour. "My iron shots were just inside 20 feet all the time, so I could just putt it out."
She hit 13 greens in regulation and needed only 27 putts in her morning round.
"Oh, it was much better this morning," Kang said. "As soon as I made the turn, it started blow. But I played yesterday the practice round with the rain, also, so I'm ready."
Australia's Karrie Webb, the tournament winner in 1995, 1997 and 2002, was a stroke back along with 16-year-old English amateur Charley Hull, Jiyai Shin, Ai Miyazato, Mika Miyazato, Stacey Keating, Lydia Hall, Vicky Hurst and Kate Kutcher.
"I think it counts for a little bit," Webb said about her experience in the event. "But you've still got to go out there and hit the shot, and you've got to commit to the lines that you want to hit your shots on. ... There's a lot of links courses that there's a side to miss on, and I don't think this course, especially off the tee, there's a side to miss on. You've just got to get up there and hit a good shot."
Shin, the 2008 winner at Sunningdale, won the Kingsmill Championship on Monday in Virginia, beating Paula Creamer on the ninth hole of a playoff.
Two-time defending champion Yani Tseng opened with a 72. She played alongside Ai Miyazato and Creamer.