The ionic conductivity of 78Li(2)S center dot 22P(2)S(5) glass-ceramics is investigated as a function of the phase and the amount of precipitated crystal produces by two-step heat treatment comprising controlled nucleation and growth stages. Crystallization kinetics of amorphous 78Li(2)S center dot 22P(2)S(5) is analyzed by differential thermal analysis, and the obtained results show that the maximum nucleation rate and crystal growth rate is achieved at 170 degrees C and 230 degrees C, respectively. The heat treatment process is optimized based on classical phase transformation theory, using the maximum nucleation rate and crystal growth rate. The optimized heat treatment consists of nucleation at 170 degrees C for 30 min followed by crystal growth at 230 degrees C for 3 h. Lithium ion conductivity is improved by 89% by using this treatment to control the crystallization kinetics. (C) 2015 Elsevier B.V. All rights reserved.