Abstract: Band gap properties of a two-dimensional square lattice steel/water phononic crystal and defect state were investigated both theoretically and experimentally.The band structure was calculated with the plane wave expansion method.An absolute band gap from 206.2 to 280.1 kHz opened between the first and the second bands.Along the two high symmetry directions (ΓΧ and ΓΜ) of the first Brillouin zone, two part gaps appeared, i.e., (138.5-280.1) kHz and (206.2-287.1) kHz, respectively.Using finite element method, transmission spectra was computed for phononic crystals with and without line defect.Some strong attenuation peaks appeared for periodic structures, and abrupt increment among the band gaps regions occurred for line defect structure.These results showed that acoustic wave localized in line defect, which were in accordance with the pressure field at 225.9 kHz acoustic wave propagating in water.Based on well-known ultrasonic immersion transmission technique, transmission spectra were measured for periodic and line defect phononic crystals.The measured transmission spectra agrees well with the numerical results.