The NASICON oxides with general formula Li1+x Y x Zr2−x (PO4)3 (0.1 ≤ x ≤ 0.2) are prepared by conventional solid-state reaction. The samples are characterized by XRD, SEM, electrochemical impedance spectroscopy and 7Li MAS NMR measurements. The structures are refined by the Rietveld method from powder X-ray diffraction data. With the introduction of Y3+, the volume of the large M1 cavity is reduced and the rhombohedral NASICON phase is stabilized at room temperature. The bulk and total Li+ conductivities of Li1.15Y0.15Zr1.85(PO4)3 sintered by SPS are 1.4 × 10−4 and 0.71 × 10−4 S cm−1 at 25 °C, respectively; the activation energy is about 0.39 eV in the temperature range 300–473 K.