The purpose of this research was to uncover the expertise of ICU nursing in Korea, and to describe nurses' practice based on the degree of skill acquisition. A total of 18 ICU nurses participated in the study. The data was collected through individual in-depth interviews and it was managed using the NUDIST 4.0 software program. The data was analyzed using interpretive phenomenology suggested by Benner. Four properties of expertise of ICU nursing were identified. These are 1) concern and love toward patients; 2) knowledge; 3) skill, and 4) abilities in interpersonal relationships. And the characterisitcs of four levels of skill acquisition, that is advanced beginner, competent, proficient, and expert, were described with exemplars. The results of this study might help nurse researchers clarify and elaborate on the concept of expertise in ICU nursing, and enable them to understand how the process of skill acquisition occurs in the ICU setting. They might also help nurse managers establish educational goals for ICU nursing for student or novice nurses.