A method is proposed to analyze the modal damping in mistuned bladed-disk with root joints using large finite element models and the detailed description of frictional interactions at contact interfaces. The influence of mistuning on the dissipated energy for different blades on a bladed-disk and the modal damping factors for different vibration levels for any family of modes can be investigated. The dissipated energy and damping factors due to microslip are simulated by multitude of surface-to-surface elements modeling the friction contact interactions at root joints. The analysis is performed in the time domain, and an original reduction method is developed to obtain the results with acceptable computational times. The model reduction method allows the calculation of the modal damping of the mistuned assembly by evaluation of the energy dissipated at root joint of each individual blade using small parts of bladed disk sectors. The dependency of modal damping factor on blade mode shapes, engine-order excitation numbers, nodal diameter numbers, and vibration amplitudes is studied and the distributions of amplitude and dissipated energy on the mistuned bladed-disk are investigated using a realistic blade disk model.