Diabetic Foot Ulcer (DFU) is one of diabetes mellitus (DM) complication that leads to amputation. It can be prevented through appropriate foot care that requires family support and helps. Family’s knowledge and efficacy are factors that support the patient’s foot care behavior. The purpose of this study was to examine the effect of the foot care education program on knowledge and self-efficacy of the family who cares the diabetes mellitus patient. This was the quasi experimental study with one group pre and post test. The respondents were the family as a care giver of diabetes mellitus patient who was hospitalized in PKU Muhammadiyah Gamping Yogyakarta Hospital. Twenty-six respondents participated in this study were selected through convenience sampling technique. The intervention that consist of education, demonstration, practice, and discussion was given individually to the family in the patient’s room within 30–60 minutes depending on the respondent’s need. The booklet and foot care package were given freely to the family. The data were collected by using family foot care knowledge questionnaire and family foot care efficacy questionnaire. Paired t-test was used to analyze the data. The result of the study showed that the respondents have the average age of 41 years and have been caring the patients for average 3 years. The respondents were the patient’s spouse and child (38,5% respectively) and never received foot care education (84,5%). The family’s knowledge and efficacy were significantly different after the intervention (p = 0,000 respectively). Foot care education program effectively improve the knowledge and efficacy of family as the care giver of patients with diabetes mellitus. Nurses should actively involve the family in diabetes foot care. Further research can examine the ability of patients and family in diabetes foot care practice and long term effect of an intervention.