Prediction of modal characteristics of acoustic cavities with irregular shapes is an important topi c in vibro-acoustic analysis of systems. In this paper, proposed method of impedance and mobility compact matrix (IMCM) provided the approach of Integro Modal Method (IMM) in the matrix form for prediction of modal characteristics of acoustical cavities with irregular shape. This method, consists of discretizing the whole cavity into a series of sub cavities either regular or irregular cavities. Acoustic pressure in regular sub cavities has been decomposed over a modal basis and for irregular sub cavities over that of bounding surface. Continuity of both pressure and velocity between adjacent sub cavities is ensured using a membrane with zero mass and stiffness. The objective of this method is to develop a physical basis rather than a numerical approach. Mathematical formulation of the method for acoustic cavities with irregular shape has been explained in detail. Predicted natural frequencies of the simplified aircraft cabin based on the proposed method have been compared with the available results in the literature. © 2016, German Acoustical Society (DEGA). All rights reserved.