An unhealthy diet contributes to the incident rate of people with type 2 diabetes mellitus (T2DM) in the world. The key to diet success is influenced by many factors and one of them is the level of family knowledge. This study aimed to assess the level of family knowledge on DM diet. The descriptive-analytic method was used by involving 45 respondents of the clustering system (sampling) with a total population of 276 people. The result of research is 53.3% of respondents were in poor knowledge. Family with age range 30–60 years had poor knowledge (37.8%), a vast of family were women (55.6%), level of education was graduated from high school (40%). Most respondents are unemployed (55.6%) and they had never known about the diabetes mellitus diet before (60%). The conclusion of the study is the family of DM patients in Kasihan, Yogyakarta had poor knowledge on DM diet. The healthcare professionals should provide more information in detail particularly on DM diet toward the family members of DM patients.