This　paper　theoretically　and　experimentally　studies　the　existence　of　complete　band　gaps　in　two-dimensional　(2D)　phononic　crystal　consisting　of　parallel　steel　rods　in　water　with　square　lattice.　The　band　structure　of　phononic　crystal　is　calculated　by　a　plane　wave　expansion　(PWE)　method.　Based　on　the　well-known　ultrasonic　immersion　transmission　technique,　the　overlapping　transmission　spectra　of　acoustic　waves,　a　complete　band　gap,　is　experimentally　measured　along　the　two　high-symmetry　directions　of　the　first　irreducible　Brillouin　zone.　There　is　a　very　good　agreement　between　the　experimental　result　and　the　range　of　frequencies　of　the　complete　band　gap.　©　2010　Higher　Education　Press　and　Springer-Verlag　Berlin　Heidelberg.