Although a large proportion of firms are family owned and most family firms are private, our understanding of private family firms is limited. Using confidential information on family relationship between board members, CEOs, and shareholders, this is the first study that provides large-scale evidence on the association between governance structure and firm performance in family controlled private firms. Our sample is unique as it covers almost all private limited liability firms in Norway, spans 11 years, traces firm ownership to ultimate owners, and identifies family relationship using data on kinship, marriage, and adoption. The results show a U-shaped relationship between family ownership and firm performance. Higher ownership of the second largest owner, higher percentage of family members on the board, stronger family power, and smaller boards are associated with higher firm performance. In addition, the positive association between the ownership of the second largest owner and firm performance also occurs when the second largest owner is a member of the controlling family, but the association is stronger when the second largest owner is a non-family member. We further test the relative importance of these test variables and find that ownership structure is more associated with firm performance than board structure.