Abstarct: This research mainly aims to introduce topology optimization as a powerful means for damage detection in three dimensional continuum problems. A procedure for Structural Health Monitoring (SHM) is proposed in this paper consisting of both system identification and damage detection parts. Mode shape baxsed objective function is considered to minimize the difference between responses of real and numerical models. For system identification, modal expansion techniques are used to obtain unmeasured data from measured data. For that aim, dynamic expansion method and TSEREP method are used in this research. In dynamic expansion method global mass and stiffness matrices are directly used to obtain unmeasured data. In TSEREP method a transformation matrix is computed by linear transformation between intact mode shapes and damaged mode shapes at measured degrees of freedom. Then transformation matrix is used to obtain unmeasured data. Afterwards, damage detection process is used to localize and quantify damages in structure. Damage is assumed as reduction in mass and stiffness of elements. In order to model damages, SIMP method is used. For topology optimization, steepest descent method is utilized to optimize objective function. In sensitivity analysis, derivatives of design variables are calculated by analytical sensitivity formulation. In the end 6 numerical examples are presented in the end to show accuracy of proposed method. According to results, it can be claimed that proposed method can obtain unmeasured data by the aid of modal expansion techniques. Also, damage detection process can localize and quantify damages accurately. Innovation of this research is to utilize modal expansion techniques in damage detection of continuum structures to overcome incomplete data issue and using topology optimization method for parameterization of design domain in order to model damage and attain damage location and intensity. For mentioned purposes, a MATLAB code was developed baxsed on 3D finite element method and results are discussed over 6 different examples provided in section five. According to the results, location and intensity of damages are obtained properly.