Objectives Family conflict has several adverse impacts on caregivers. Thus, there is significant value in determining the prevalence and predictors of family conflict, which can enable the health care provider to intervene if family conflict arises during end‐of‐life care. Accordingly, we aimed to explore the prevalence and predictors of conflict among the families of patients with advanced cancer who died in palliative care units. Methods This study was a nationwide multicenter questionnaire survey of bereaved family members of cancer patients who died in Japanese palliative care units participating in evaluation of the quality of end‐of‐life care. Results We sent out 764 questionnaires, and 529 questionnaires (69.2%) were returned. As 70 family members refused to participate and we could not identify the answers in one questionnaire, we analyzed a total of 458 responses. The average Outcome‐Family Conflict score was 13.5 ± 4.9 (maximum score: 39.5), and 42.2% of family members reported at least one family conflict during end‐of‐life care. Greater family conflict was significantly associated with younger family age, with family members asserting control over decision making for patient care and with communication constraints among family members, although absent family members “coming out of the woodwork” reduced family conflict. Conclusions Many families of patients with advanced cancer experienced conflict during end‐of‐life care. Family members asserting control over decision making and communication constraints among family members after diagnosis of cancer can predict the occurrence of family conflict. Absent family members “coming out of the woodwork” might reduce family conflict in particular cultures.