||In this thesis, I present the fabrication process of three-dimensional (3D) solid-in-solid phononic crystals (PCs) and experimentally studied their ultrasonic properties by measuring their underwater transmissions. Most of the PCs are in body-centered-tetragonal (BCT) structure with spherical scatters. They can be cataloged into three general types by the matrix material: paraffin-matrix, polystyrene-matrix and Aluminum-matrix. In all three types, full band gaps were observed in the transmission coefficient curves, at least for the longitudinal modes. The 1st type of PCs exhibit the widest gap while the 3rd type exhibit the narrowest gap. Different sizes of scatters were used for verification of the gap location and width using the scalability principle in the 1st and 2nd types of PCs. And phase curves provide additional information in confirming the gap existence in the 3rd type of PCs, most of which are composed of 1mm-diameter tungsten carbide (WC) beads embedded in Aluminum (Al). Both defect states and collimation were experimentally demonstrated for the first time in the 3D solid-in-solid type of PCs. In the WC/Al PCs, defect states were experimentally demonstrated in the gap regime, by introducing a cluster defects of Si3N4 beads of the same size. It was confirmed by both the spectral curve and the transmitted pressure field patterns. Collimation occurring in the 2nd pass band was also recognized in mapping the transmitted pressure field patterns through a WC/Al PC doped with 3D cluster defects of Si3N4 beads of the same size. Multiple Scattering Technique (MST) was used to calculate the band structures of the undoped PCs in BCT. Some discrepancies are explained by two-dimensional PCs simulations using Comsol.