The topic of this paper is system identification of structures conducted with　consideration of incomplete modal information. A continuum structure has infinite number of　degrees of freedom and an infinite number of modes. Modal estimation method is therefore　not　employed to identify all modes of systems. In addition, incomplete modal information may be　led　from insufficient response data in practical vibration measurement. In this paper, by using　channel-expansion technique, modal estimation from response data of insufficient channel　can　be　performed through the Ibrahim time-domain method. Applicability and effectiveness of the　proposed method is demonstrated by numerical simulation of a chain model. Identification of　the　mode shapes, however, is still a challenging problem to be resolve in the proposed method from　incomplete modal information data.