Purpose: The purpose of this study was to identify the factors related to the functioning of family caregivers of patients with cancer. Method: Data were collected by questionnaires from 124 patient-family caregiver dyads at a hospital in Seoul. Data collection was done between August, 2004 and January, 2005. Data were analyzed using Pearson correlation coefficients and stepwise multiple regression. Results: The mean score for family functioning was 68.73. Family functioning showed a significant negative correlation with burden of family caregiver and performance status of patients with cancer, and a significant positive correlation with previous relationship between the patient with cancer and caregiver. The most powerful predictor of family functioning was the relationship between the patient and caregiver. The relationship between the patient with cancer and caregiver, and performance status of the patient accounted for 25.4% of the variance of family functioning. Conclusion: The results showed that the relationship between patients with cancer and caregivers and performance status of patients with cancer were significant factors influencing family functioning in family caregiver of patients with cancer.