Molecular dynamics simulation was performed to calculate the ionic conductivity of yttria-stabilized zirconia (YSZ), by varying Y2O3 concentration to 5.9, 8.0, 10.2 and 12.5 mol%. The calculated conductivity exhibited a maximum at 8 mol%, which agreed with previously reported experimental data. In order to clarify the reason for this maximum conductivity, the position occupied by oxygen vacancies was determined. It was found that oxygen vacancies are trapped at the second neighbor position to Y ions. It was also found that the number of oxygen sites which trap the vacancy increased with increasing Y2O3 concentration. Therefore, the trade-off effect between the increase in the number of vacancies and the decrease of vacancy mobility was considered to be the reason for the maximum ionic conductivity occurring at a Y2O3 concentration of 8 mol%.