Abstract: 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.