The ionic conductivity of 78Li2S·22P2S5 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 78Li2S·22P2S5 is analyzed by differential thermal analysis, and the obtained results show that the maximum nucleation rate and crystal growth rate is achieved at 170 °C and 230 °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 °C for 30 min followed by crystal growth at 230 °C for 3 h. Lithium ion conductivity is improved by 89% by using this treatment to control the crystallization kinetics.