The effect of embedded channels and cavities on the strength of LTCC substrates has been investigated by experiments, theoretical analysis and finite element analysis (FEA) simulation. The fracture behaviors characterized by flexural strength were measured by three-point bending tests and discussed according to the features of the microstructure, which was studied by Scanning Electron Microscope, Energy Spectrum Analysis and X-Ray Diffraction methods. The experimental results show that X and Y-axial channels have little effect on the flexural strength while Z-axial via-hole will greatly lower the strength due to the stress concentration. Two distinct components were observed: particles composed of synthetic corundum and matrix composed of corundum, silica, aluminum, and sodium calcium aluminum silicate. The FEA simulation gave similar results to the experiments and theoretical calculations and proved to be an effective method to predict possible condition of the fracture on substrates with complex internal structures.