The role of the acute care nurse practitioner in Taiwan has changed significantly since the 1990s due in significant part to a shortage of interns and resident physicians in acute care settings. The first year of professional practice represents an important transitional year during which new professionals develop their competency to provide high-quality care to hospitalized patients. As the actual experience of individuals undergoing this transitional process into their new role has yet to be fully explored, this research studied and categorized the experiences of acute care nurse practitioners during their first year of role transition under the collaborative model of practice. We used a qualitative inquiry method with in-depth interviews to investigate the relevant experiences of 10 acute care nurse practitioners working at a medical center in Taiwan. Results show that the experience of expert nurses in their transition to acute care nurse practitioners passed through three phases during the first year under the collaborative practice model. These phases include the role ambiguity, role acquisition, and role implementation phases. Each phase contained a set of sub-themes which describe the multiple dimensions of this experience. This study highlights the experiences, stresses and accomplishments of acute care nurse practitioners during their initial year of advanced practice.