Vibration in cutting platform of corn harvest combine causes increase in grain loss, reduction of vehicle lifetime and driver’s comfort, and also affects the machine working precision. In order to determine linear vibration behavior of a mechanical structure, a model which relates acting forces on the structure with the resulting responses of it plays a significant role. For a great number of mechanical structures, only response data are measurable while the actual loading conditions are not known. Operational Modal Analysis (OMA) is a process of determining dynamic characteristics of a structure based on the output responses only (i.e. input excitation information is not required). The use of such processes allows the identification of modal models of structures which are excited by unknown ambient force and vibration. In this thesis, controlled vibration tests are conducted on cutting platform of a John Deer 955 combine. During the test, the combine is excited by its engine and the resulting vibrations of different working sections, such as the trashing mechanism, elevator and cutter bar. The acceleration signals are recorded with a 6-channel data acquisition device associated with PULSE Lab Shop measurement software. The PULSE system is regulated for sampling interval of 1.953 ms. Therefore, about 16384 data points are acquired, corresponding to a measurement time of 32 s. Measurements are taken for constant values of the engine speed and the cutting height. Next, the frequency domain decomposition (FDD) technique is used by the MEscope software to estimate the modal parameters of vibrating structure in real operational conditions. FDD technique is a non-parametric technique for operational modal analysis (OMA) of structures that estimates the modal parameters directly from signal processing calculations. Then, a finite element model is constructed representing the vibration behavior of the cutting platform. For this purpose, the body of platform, knife, spiraled auger, reel and a total of 56 bolts, 13 knife clips and 28 guards are modeled in ABAQUS. These parts are assembled by 140 coaxial constrains and 100 face to face constrains and meshed automatically with tetrahedron elements. Then, the finite element model is successfully updated by comparing the estimated results with the measured data from operational modal analysis. Also, a resonance condition is found around the natural frequency of the fifth mode shape (50 Hz). Since the scaling of estimated operational mode shape depends on the unknown level of the ambient excitation, an extra effort is required in order to correctly merge the different parts of the mode shapes. Therefore, the mass change strategy is used to estimate the scaling-factor. The FRF matrix is then calculated by scaled mode shapes. Finally, the modification technique is introduced on the structure by change of mass matrix in order to achieve better harvest performance and have easier and more comfortable operation. The structural modification methods often prove to be the best and sometimes the only strategy for the redesign of complex structures. As a result of this modification, natural frequency of fifth mode shape (50 Hz) of the combine cutting platform was shifted to 48 Hz. Thus the resonance condition and vibration of cutting platform is reduced. Keyword : cutting platform, FDD technique, operational modal analysis, mass modification.