The combination of sol-gel synthesis and high energy ball milling is used to investigate the phase transition from tetragonal Li7La3Zr2O12 to cubic Li7La3Zr2O12 without subsequent high-temperature calcination, typically above 1200 degrees C. In this report, tetragonal Li7La3Zr2O12 powders obtained by sol-gel method and low-temperature thermal treatments were treated in ethanol with agate vials using a conventional high-energy planetary ball mill. For the sample through sol-gel transition and calcined at 900 degrees C for 6 h, X-ray diffraction measurement revealed that the ball milling treatment induces a direct phase transition from tetragonal to cubic Li7La3Zr2O12 without high-temperature heat treatment. However, in a comparative study, cubic Li7La3Zr2O12 can only be obtained after high-temperature calcination above 1180 degrees C using the same precursor without ball milling treatment. We hypothesize that the high energy ball milling promotes the accumulation of Li+ vacancy leading to this easy phase transition. This study establishes an efficient way to transform tetragonal Li7La3Zr2O12 to cubic Li7La3Zr2O12 at lower calcination temperature via facile high energy ball milling treatments.