Miyazato, who won the event in 2010, is one stroke ahead of five players and two up on two others.
Choi Na-yeon, Anna Nordqvist, Pak Se-ri, Karrie Webb and Amy Yang are tied for second at 4-under. Nordqvist and Webb had four birdies on the day while Choi, Pak and Yang each had five birdies and one bogey.
Christel Boeljon, after six birdies and three bogeys, and Numa Gulyanamitta, who had five birdies and two bogeys, are tied for seventh but just two strokes off the lead.
Miyazato opened with a birdie and made the turn at 2-under after a birdie at the ninth hole. She moved into the lead with three second-nine birdies, including at Nos. 12 and 16, which are par 3s at the Siam Country Club.