Thermoelectric Zn4Sb3 prepared by spark plasma sintering (SPS) method has been studied by X-ray diffraction (XRD) and positron annihilation spectroscopy. A reversible structural transformation between beta-Zn4Sb3 and ZnSb was confirmed by XRD measurements. It was found that the as-milled powder is composed of the pure beta-Zn4Sb3 phase, but after SPS, one side of the sintered sample changes to the ZnSb phase. Upon annealing at 300 A degrees C, the ZnSb phase converts back to beta-Zn4Sb3 phase. During these phase transition processes, Zn migration driven by direct current and Zn diffusion activated by annealing play important roles. Another interesting finding is that excess Zn can suppress the decomposition of beta-Zn4Sb3 into ZnSb and Zn. Positron annihilation lifetime measurements confirm that the diffusion of Zn introduces large amounts of vacancy clusters in the ZnSb-side, and most of them are recovered when the ZnSb phase converts back to beta-Zn4Sb3 due to the migration of Zn driven by annealing. The positron lifetime results further show the acceleration of Zn migration at about 150 A degrees C. The existence of Zn vacancies is also observed in the beta-Zn4Sb3 structure.