An organized numerical and experimental study is carried out in order to understand ultrasonic guided wave (GW) propagation and interaction with a high density (HD) core region in a honeycomb composite sandwich structure (HCSS). Also the location of HD core region in a HCSS using embedded piezoelectric wafer transducer (PWTs) is investigated in this study. Due to complex structural characteristics, the study of guided wave (GW) propagation in HCSS with HD-core region inherently carries many challenges. Therefore, a three-dimensional (3D) numerical simulation of GW propagation in the HCSS with and without HD core region is carried out using embedded PWT network. It is observed that the presence of HD core significantly reduces the amplitude of the propagating GW modes. In order to verify the numerical results, experiments are conducted in the laboratory. A good agreement between the numerical and experimental results is observed, in all the cases studied. Finally, based on the change in amplitude of the received GW modes, the location of an unknown HD core region, within the PWT array is determined by applying a probability based signal difference coefficient (SDC) algorithm.