With regard to moment-resistant frames composed of CFT (concrete-filled tube) columns, H-shaped beams and diagonal braces, the strong axial force of a diagonal brace firstly transfer to the steel tubular portion of a column at beam-to-column connection and then to the filled concrete. Two types of stress transfer mechanisms from steel portion to the filled concrete may be available in CFT columns. The one is the concrete bond in a steel tube, and the other is a mechanical shear key effect functioned by the cut-through diaphragms which are often used for a beam-to-column connection in Japan. However, either of the mechanisms have not been demonstrated by realistic frame assembly specimens so far. Such a lack of the information is now causing the over-safety design for the stress transfer in a connection especially installed by diagonal braces. From this point of view, we performed a horizontal loading test of CFT frame assemblies, and discussed the stress transfer effects for both mechanisms.