Objective: The vital role played by primary caregivers in caring for cancer patients is well‐recognized, but the caregiver burden and impact on family functioning to caregivers’ mental health is poorly understood. This study examined the prospective and reciprocal relationships between family functioning, caregiver burden, and mental health. We aimed to determine whether inferior family functioning and heavy caregiver burden act as risk factors for mental health, as consequences of mental health, or both.