Thermoplastic starch (TPS) and polyvinyl alcohol (PVA) blends were prepared by extrusion method in the presence of water/glycerol and citric acid, and their phase behavior was investigated. Results of scanning electron microscopy (SEM) and X-ray diffractometry (XRD) tests demonstrated the complete plasticization of starch during the melt mixing. PVA crystal phase was not formed in blends with less than 50 wt% PVA. Fourier-transform infrared spectroscopy (FTIR) and dynamic mechanical analysis (DMA) analysis revealed high compatibility between two phases, starch and PVA. Addition of PVA to TPS resulted in the reduction of the glass transition temperature and dynamic moduli and improvement of sample flexibility. However, at 10 wt% PVA, an anti-plasticization effect caused to increase the Tg and storage modulus of the blend.