The number of deaths of older adults in long-term care settings will increase with the aging population. Nurses and care workers in these settings face various challenges in providing end-of-life care, and interventions for quality end-of-life care may be useful. This feasibility study aims to explore the preliminary effect and acceptability of an intervention named the EOL Care Tool to improve end-of-life care in long-term-care facilities. We conducted a single-arm quasi-experimental study using mixed methods. This tool consisted of multiple components: professionalized lectures, newly developed structured documents, regular conferences regarding end-of-life care, and educational support from administrators. Twenty-four nurses and fifty-five care workers employed in a long-term care facility participated. For nurses, improvement in attitudes toward end-of-life care (p < 0.05) and interdisciplinary collaboration (p < 0.05) were shown quantitatively. Regarding acceptability, nurses and care workers evaluated the tool positively except for the difficulty of using the new documents. However, qualitative results showed that care workers felt the reluctance to address the work regarding end-of-life care. Therefore, a good preliminary effect and acceptability for nurses were indicated, while acceptability for care workers was only moderate. Revision to address the mentioned issues and evaluation of the revised tool with a more robust research design are required.