Finite element model has great importance in mechanical and structural engineering (such as automobile, aerospace and structural engineering industries). The main purpose of the finite element model is to provide a reliable numerical model of the system and predict the system response to different disturbances. However, due to discretization, idealization and simplification, there is substantial discrepancy between the test measurements and the corresponding values associated with the nominal finite element model. The purpose of model updating is to enhance the nominal model, using test measurements of the structure, to reflect better the reality. Various algorithms have been developed to achieve this goal by using modal data (i.e. identification results of modal frequencies and mode shapes). In this thesis, a novel direct finite element model updating method is proposed by using incomplete modal data. The method is inspired from system control theory and it updates the structural model to possess the target modal frequencies and mode shapes. The incomplete mode shapes of the observed modes are expanded to full scale and the correlation between the updated and actual mode shapes is improved. Furthermore, the modal frequencies and mode shapes of the unobserved modes will be retained. Computational efficiency of this method enhances its applicability to large structures since it requires neither iteration nor nonlinear optimization process. Moreover, there is no need to compute the modal frequencies or mode shapes of the unobserved modes for the nominal or other models. Examples of an 18-story building and a 7200-degree-of-freedom space frame structure are used to demonstrate the accuracy and efficiency of the proposed method. It requires less than 16 seconds for the updating of this 7200-degree-of-freedom structure by using a usual desktop computer. Structural responses are also computed for both the 18-story building and the 7200-degree-of-freedom space frame structure. Results demonstrate the reliability and efficiency of the proposed finite element model updating method