Abstract: We present the analytical calculation of transcendental equation obeyed by quasi-bound energy levels of electron for motion perpendicular to interfaces of the Quantum Well and Nanostructure Physics of non-isolated Quantum Well of symmetric rectangular double barrier. In this investigation we need to calculate expressions of elements of transfer matrix of the left hand rectangular tunnel barrier (TL matrix) and expressions of elements of inverse transfer matrix of the right hand rectangular tunnel barrier (ITR matrix) to get the desired calculation.