Longitudinal vibration mode is conveniently used for damage localization and detection in truss structure. However, a satisfactory piezoelectric impedance response due to longitudinal modes is not so easy to obtain in practical applications. The objective of this paper is to investigate how to obtain the impedance response of longitudinal modes with a piece of PZT transducer. In this case various vibration modes will be excited by unilaterally bonded PZT transducer, so the observation of longitudinal modes becomes complicated. In this paper, the effect of parameters of PZT transducer such as the length and attachment position is discussed in detail. Through selecting a suitable size and attachment position, the impedance response of expected longitudinal modes can be generated. An example for generating impedance peaks of specified longitudinal mode in fixed-fixed beam is given to demonstrate its validity.