The article presents the results of the development and testing of methodology for evaluation of the well-being of children living in state care institutions for orphans and children left without parental care. Evaluation of the well-being of children is carried out from the perspective of the child: children are the key informants in the evaluation; the questionnaire covers topics that are important for subjective well-being of the child. The child-friendly questionnaire covers the following self-reported domains of well-being: mood and emotional state, state of health, physical activity, education, relationships with other children and adults, safety, living conditions in the care institution, participation in decision-making and knowledge of rights. Evaluation of validity and reliability of the 22 items well-being scale using a data from 517 orphan children living in 21 state care institutions in three Russian regions was conducted. Organization of adequate conditions of data collection was rather challenging in the context of state care institutions. Generally, children gave highly positive feedback on the questionnaire layout and content and opportunity to be heard. Implications of these results for future research and evaluation of children's well-being in state care institutions are discussed.