The ionic conductivity of the Li2S-P2S5 glass electrolyte systems strongly depends on the type of precipitated crystalline phases and heat-treatment conditions. To clarify the correlation between ionic conductivity and crystallization behavior, a specific observation technique to visualize the precipitated nanocrystallites via transmission electron microscopy (TEM), and a quantitative method for evaluating the crystallization degree are developed. The 75Li(2)S center dot 25P(2)S(5) glass exhibits a high conductivity of approximately 7.5 x 10(-4) S/cm at a sintering temperature between 120 degrees C and 180 degrees C. Above 180 degrees C, however, conductivity sharply decreases to 1.8 x 10(-4) S/cm. In situ TEM observation reveals that this is attributed to the crystallization of the glasses. The crystallization process is mainly characterized by the formation of beta-Li3PS4 nanocrystallites precipitated above 180 degrees C whose size remains intact during further heating until 250 degrees C. As a result, to achieve high ionic conductivity in the 75Li(2)S center dot 25P(2)S(5) glass, it is important to realize an amorphous state with the presence of nanocrystallites that would yield a crystallinity degree lower than 5%.