Aim: Family caregivers have limited abilities that make them vulnerable to the care needs of patients. Therefore, it seems necessary to evaluate their caring ability. The aim of this study was to design an instrument for assessing the caring ability of family caregivers of cancer patients.; Methods: This was a sequential exploratory mixed-method study, carried out in two qualitative and quantitative phases. The concept of caring ability and its dimensions were explained using conventional content analysis in the qualitative phase.