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In this dissertation, the interval modal analysis method using NASTRAN DMAP language, to estimate the bound of natural frequency of structures with uncertain-but-bounded parameters, is presented. Although the system contains uncertain parameters or properties rather than certain ones in many engineering, it can be pre-estimated the ranges of uncertain structural parameters between upper and lower bound by practical experience and engineering knowledge. Moreover, in order to avoid the resonance of a structure and to consider the dynamic response of uncertain one, for the reliability and stability analysis in practical engineering, we often need the bounded range of structural characteristic parameters such as natural frequency, normal mode, etc. Herein, by using the modal analysis and interval calculus via NASTRAN software, it is investigated the method for computing the upper and lower bounds on natural frequencies and its modal shapes of structures with bounded uncertain (or interval) parameters.In this dissertation, in order to save the analysis time and to improve the accuracy of analysis results, it is used the DMAP language as high level application to use NASTRAN source subroutines for structural analysis. In past time, it had been used by oneself edited program or FORTRAN source programs for its structural analysis in direct-use-difficult analysis part on the structural analysis software (such as ANSYS, NASTRAN, etc.). These methods need to understand well about known FORTRAN subroutines for structural analysis and its consumptive time was comparatively long, and it was also difficult to believe its results because of neglecting an ignorant program part and other errors or mistakes in applying the known FORTRAN subroutines.In Chapter 2, it was introduced some basic knowledge of the modal analysis and interval mathematics, briefly. Chapter 3 shows the interval eigenvalue analysis, the interval perturbation analysis and the interval modal analysis method that is proposed in here, in order to determine the bound of natural frequency of structure. Numerical example is presented in Chapter 4, where its results illustrate that the proposed methods are effective. A comparison of interval eigenvalue analysis methods with the results from the Monte Carlo simulation and the modal testing analysis, serves to validate the solutions and identify their bounded ranges.