The aim of this paper is to investigate the bandgap properties of a piezoelectric phononic crystal (PC) nanobeam with size effect by coupling the plane wave expansion method, Euler-Bernoulli beam theory and nonlocal theory. The first four orders were chosen to study the influences of thermo-electro coupling, size effect and geometric parameters on band gaps. Temperature change and external electrical voltage were chosen as the parameters capable of influencing thermo-electro coupling fields. Scale coefficient was chosen as the influencing parameters related to size effect. The lengths of PZT-4 and epoxy within a unit cell, along with the width and thickness of the PC nanobeam, were identified as influential geometric parameters. Collectively, our results are expected to be helpful for the design of piezoelectric nanobeam-based devices.