This paper is estimated the damage of truss bridge based on System Identification(S.I.) using modal test data. Generally, civil structures mainly require sophisticated finite element models having a number of degrees of freedom, also in experiments and measurements on structures. The sparseness and errors of measured data have to be considered during the damage assessment of structures. To study the behaviour of the estimators in noisy environment using Monte Carlo simulation, and a data measured perturbation scheme is adopted to investigate the influence of measurement errors on identification results. The assessment result by equation error function and response error function were compared, and the efficiency and applicabilities of the proposed algorithm are demonstrated through simulated modal test data of a truss bridge.