The crystallization process of the 70Li(2)S center dot 30P(2)S(5) (mol%) glass was investigated to obtain a guideline for enhancing the crystallinity of Li7P3S11 crystals in a glass-ceramic. Results show that the glass softened at around 240 degrees C and that surface crystallization of the glass started at around 250 degrees C. The heat-treatment conditions with a maximum nucleation rate for the glass were estimated using DSC measurements. The glass-ceramic prepared using two-step heat treatment of 210 degrees C for 30 min (nucleation) and 280 degrees C for 1 h (crystallization) exhibited high conductivity of 5.2 x 10-3 S/cm at room temperature.