The aim of this thesis is the estimation of errors due to reduction in modal superposition method. Mode superposition methods are used to calculate the dynamic response of linear systems. In modal superposition method, it becomes unnecessary to consider all the modes of a particular system. In generally only a few number of modes contribute significantly to the solution. The main aim of the study is to identify the significant modes required for good approximation. Finally, this study constitutes a basis for developing an adaptive method, choosing the minimum number of modes required for certain accuracy. Procedure for evaluating errors is presented and numerical examples illustrate the behavior of the method in the context of forced harmonic response. Finite element modeling and simulation was carried out using MATLAB.