The　band　structure　of　two-dimensional　(2D)　phononic　crystal　that　consists　of　a　honeycomb　array　of　steel　cylinders　in　water　is　calculated　by　the　plane　wave　expansion　(PWE)　method　and　structure　factors　of　the　phononic　crystal　are　deduced　in　detail.　Based　on　the　ultrasonic　immersion　transmission　technique,　the　directed　band　gaps　are　experimentally　measured　along　the　two　high-symmetry　directions　of　the　first　irreducible　Brillouin　zone.　Experimental　result　of　frequency　extending　of　the　first　complete　band　gap　agrees　well　with　the　theoretical　calculation.