Based　on　the　ultrasonic　immersion　transmission　technique,　the　directed　band　gaps　of　two-dimensional　(2D)　phononic　crystal　consists　of　a　square　array　of　steel　cylinders　in　water　were　measured　experimentally　along　the　Γ-X　and　Γ-M　directions　of　first　irreducible　Brillouin　zone.　The　relations　of　layers　and　first　band　gaps　stability　can　be　analyzed　in　detail.　This　is　better　advantage　of　design　for　finite-sized　2D　phononic　crystal　or　acoustic　devices.　An　excellent　agreement　between　the　frequency　range　of　overlapping　transmission　spectra　of　acoustic　waves　and　complete　band　gap　of　2D　phononic　crystal　for　theoretical　predict　with　plane　wave　expansion　method　is　observe.