Acoustic band gaps in two-dimensional phononic crystal consisting of a finite-sized square array of parallel hollow aluminum cylinders in air are investigated with the finite difference time domain (FDTD) method. Experimental measurements were carried out to validate the theoretical predictions with excellent overall agreement achieved. The numerical method is used subsequently to study the band gap properties of phononic crystals composed of cylinder (both solid and hollow)/fluid systems for two different acoustic impedance ratios. For both types of system, the influence of the transverse size of the phononic crystal on the propagation properties is analyzed, and the minimum size to achieve the largest band gap width for different filling factors is determined.