Nonlinear 3D FE is utilized to investigate the nonlinear behavior of RC internal and external joints of frames during cyclic loading. The factors affecting the behavior of RC joints include; dimensions and details of joints, main reinforcement ratios and shear reinforcement ratios of beams and columns and presence of slab. The items of the behavior such as; the hysteresis loops and the envelope curves of joints, shear strength, flexural shear strength, ductility level, and comparison between behavior of external and internal joints. The study showed the necessity of applying the weak beam – strong column concept on design of such joints. The plastic hinges should occur in the beams not on the columns to avoid brittle failure during earthquake. Presence of slab caused an increase and improvement of both strength and ductility.