Abstract Garnet-type solid electrolyte Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (LLZT) was densified by using a spark plasma sintering (SPS) technique. Formation of impurities in the obtained pellets was studied in detail. It is revealed that impurities are strongly related to the SPS process: electrolysis of LLZT and electromigration of graphite. The electrolysis results in La 2 Zr 2 O 7 on anode of the SPS process, which is accompanied by reduction of Li 2 CO 3 to amorphous carbon on cathode. The electrolysis on SPS could be successfully suppressed by employing LLZT powder without Li 2 CO 3 . When these impurities were removed, pellets obtained by SPS exhibited electrochemical performance comparable with those densified by other methods total ionic conductivity of 6.9 × 10 −4 S cm −1 at 298 K and short-circuit prevention up to 100 μA cm −2 on dc polarization. The results confirm great advantage of SPS on manufacturing the dense garnet-type solid electrolytes: a low-temperature (900–1100 °C) and short-sintering-time process (10 min).