Palliative prognostic index (PPI) is developed and utilized to assess the prognosis of terminally ill patients. Understanding the life expectancy is critically important for patients and their families. We present two terminally ill cases due to cancer in which we used prognostic evaluation tools during advance care planning (ACP) among patients, families, and medical staff. In the first case, the patient and her family initiated an ACP early in her diagnosis. The unfavorable prognoses indicated by PaP and PPI supported and reaffirmed their decision for no intensive treatment or life support, allowing her to pass away peacefully at home. The second case involves a family initially reluctant to accept the patient's ACP, which refused intensive therapy or life support for malignant tumors. However, the family understood and accepted the patient's initial ACP through careful discussions utilizing these prognostic tools. These cases highlight the importance of using objective prognostic prediction tools to support clinical judgments, particularly in the terminal stages of malignant tumors.