Calibration test is carried out to check the perfectness of the main rolling equipments on hot rolling mill stands. In calibration test rolls are loaded to the forces in the same order of rolling force for few seconds and then to the half of the mentioned force for few more seconds, while no strip is fed between the rolls. Calibration test should be performed after every roll change or after failure of computer systems. Rolling conditions can be estimated from the test results and this test can be used as a powerful tool to judge whether rolling process is permissible or not.The main goal of this research is to distinct the structural defects of the mill from force vibrations which are recorded by Ida analyzer during calibration test. As the vibration diagrams of Iba Analyzer show constant amplitude, it is concluded that forced vibration was occurred on the mill.For approach to this goal mathematical model are represented and then differential equations are derived. Structural model consists of two sub-models; vertical model and torsional model. Vertical model has two degrees of freedom and torsional model has one degree of freedom. Equations of motion for vertical model include linear vertical stiffness coefficient and linear vertical damping coefficient. Equation of motion for torsional model includes equivalent mass moment of inertia, equivalent torsional damping coefficient and equivalent torsional stiffness coefficient. At next step structural defects affect on constituted model. Structural defects consist of two items: Some structural defects affect on vertical model and the defects of them were shown by forces such as eccentricity on rolls, ovality on rolls and various defects in bearings and some structural defects effect on torsional model and the defects of them were shown by moments such as misalignment in gear couplings, various defects in gears and rolling resistance. Then ultimate equations which are coupled with each other were solved using Simulink tool box of and Matlab software. Results of simulations were compared with real data from Iba Analyzer. Comparison were done using FFT technique to find peak vibration amplitude on any oscillation frequency.Conclusions are:Forced vibration due to hardware errors are the source of the observed oscillations on Iba data.Defects which affect vertical sub-model such as eccentricity and ovality of the rolls are detectable by the calibration test. Defect amplitude could be calculated using the method which is presented on this thesis.In reverse structural defects which appear as oscillating moments and affect the tortional sub-model does not have considerable effect on force vibration in calibration test. Examples of these defects are; misalignment in gears, couplings, shafts etc. Key Words: Calibration test, force vibration, structural defect, hot rolling mill, rolls