 In 2001, Ko teamed up with actor Lee Sung-jae in "A Day", about a married couple who have trouble conceiving a child.Her acting in the film was much praised and garnered the Best Actress prize from the local Grand Bell Awards ceremony.Then, after two years off, Ko returned in 2003 opposite Han Suk-kyu in the spy thriller "Double Agent"; however, the film failed to live up to the expectations of most viewers and critics.