Using analytical formula to estimate a complex problem of cavity based on a transmission line approach is called the ETL (Equivalent Transmission Line) method . Operating frequency of dominant mode of standard case is several hundreds of megahertz that is lower than frequency of CPU, higher-order mode must be considered . Several papers always discuss a single aperture that does not correspond to practical array of apertures of case .Array of apertures can be estimated using analytical formula and higher-order mode can be considered in this paper .Usually array of apertures is not placed at the center of metallic plate, we can modify the analytical formula to get ETL of shifted array of apertures. To consider industrial design and esthetics, many possible shape of array of apertures are discussed by adapting to the ETL method .The problem of how to improve the shielding effectiveness of array of apertures is treated in the latter half of this thesis. The FDTD (Finite-Difference Time Domain) method is a full-wave analytical algorithm and is not efficient for analyzing cavity. Advantage of ETL is that it is faster than full-wave analytical algorithm .However, traditional ETL can not get accurate level of shielding effectiveness without higher-order modes. Thus improved ways are introduced in the paper to simulate practical array of apertures to reach the result of good agreement and short computing time.