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　(ΓX　and　ΓM)　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.