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. The research participants included 41 family caregivers of cancer patients and professional caregivers who were selected using purposeful sampling method until reaching data saturation. The scale items were designed using the results of the qualitative phase of the study, as well as the review of relevant literature. In the quantitative phase, the scale was validated using content and face validity, construct validity, as well as internal consistency and stability.; Results: The primary item pool was prepared in 108 items. Content validity was determined using CVR with a cut-off point (0.62), CVI with a cut-off point (0.8) and kappa coefficient (κ) (>0.75). The validity of 72 items was confirmed. Then, the overlapping items were merged and eventually the 45-item scale entered the face validity stage and five items with an impact factor < 1.5 were omitted. Results of KMO = 0.904 and Bartlett = 6184.012 (p < 0.001) justified the need for factor analysis. Scree plot indicates five factors with eigenvalues above 1 and 67.7% of the total variance, including 'Effective role play, Fatigue and Surrender, Trust, Uncertainty, and Caring ignorance'. Reliability of the 31-item instrument indicated a Cronbach's alpha coefficient of 0.93 and ICC of 0.94.; Conclusions: Caring abilities scale (CAI) of family caregivers of patients with cancer is a valid and reliable instrument that can assess caregivers' caring ability.