The ionic conductivity of Li x B1−x/3PO4 is investigated in terms of defect models with a lithium doping level of 0<x<0.3. The crystalline solid electrolytes, Li x B1−x/3PO4, are synthesized by a soft-chemical route by using H3BO3, H3PO4, LiOH·H2O. To determine the changes in ionic conductivity and activation energy of Li x B1−x/3PO4 with lithium content x, AC-IS measurements are performed at temperatures between 25 and 90°C. The maximum ionic conductivity is 3.74×10−6 Scm−1 at room temperature for x=0.10, and the activation energy increases from 0.3039 to 0.3385eV with increasing lithium content.