The glass-ceramic composite electrolytes based on tetragonal Li7La3Zr2O12 (t-LLZ) and cubic Al-doped Li7La3Zr2O12 (c-LLZ) with the LiPO3 glass additive have been prepared. The electrical conductivity and microstructure of the t-LLZ/LiPO3 and c-LLZ/LiPO3 composites have been investigated. The phase evolution of electrolytes has been studied using XRD, SEM, and Raman spectroscopy. It was indicated that the impurities formation depends on the composition of the composite. The phase composition of the solid electrolytes determines their thermal properties, which have been studied by the DSC method. The relative density of the obtained composite electrolytes was established to be higher than one of the sintered t-LLZ and c-LLZ. The Li+ conductivity of the t-LLZ-based composites gradually increased from 4.6 × 10-7 S cm-1 (undoped t-LLZ) to 2.5 × 10-6 S cm-1 (t-LLZ/5 wt % LiPO3 composite) at 25 °C. The highest total conductivity of the c-LLZ/LiPO3 composites has been achieved by introducing 1 wt % additive (0.11 mS cm-1 at room temperature), whereas further doping resulted in the impurities formation.