In this work,we experimentally investigate band gap behaviors of a phononic crystal beam.Specifically,defect modes and their influences on the transmission characteristics are studied.The phononic crystal beam is composed by two materials periodically glued to each other.The defects are introduced by damages or replacement of one material to a third material.We first investigate the band gap structures and the transmission characteristics of the phononic crystal beam by the transfer matrix method (TMM) and compare its results with those obtained by the finite element method.The band gap structures with defect modes are then theoretially and numerically compared and the sizes or locations of the defects in the phononic crystal beam are studied and the corresponding defect states are discussed.