The yttria-stabilized zirconia (YSZ) system has been extensively studied because of its critical applications, like solid oxide fuel cells (SOFCs), oxygen sensors, and jet engines. However, there are still important questions that need to be answered and significant thermodynamic information that needs to be provided for this system. There is no predictive tool for the ionic conductivity of the cubic-YSZ (c-YSZ), as an electrolyte in SOFCs. In addition, no quantitative diagram is available regarding the oxygen ion mobility in c-YSZ, which is highly effective on its ionic conductivity. Moreover, there is no applicable phase stability diagram for the nano-YSZ, which is applied in oxygen sensors. Phase diagrams are critical tools to design new applications of materials. Furthermore, even after extensive studies on the thermodynamic database of the YSZ system, the zirconia-rich side of the system shows considerable uncertainties regarding the phase equilibria, which can make the application designs unreliable. During this dissertation, the CALPHAD (CALculation of PHase Diagrams) approach was applied to provide a predictive diagram for the ionic conductivity of the c-YSZ system. The oxygen ion mobility, activation energy, and pre-exponential factor were also predicted. In addition, the CALPHAD approach was utilized to predict the Gibbs energy of bulk YSZ at different temperatures. The surface energy of each polymorph was then added to the predicted Gibbs energy of bulk YSZ to obtain the total Gibbs energy of nano-YSZ. Therefore, a 3-D phase stability diagram for the nano-YSZ system was provided, by which the stability range of each polymorph versus temperature and particle size are presented. Re-assessment of the thermodynamic database of the YSZ system was done by applying the CALPHAD approach. All of the available thermochemical and phase equilibria data were evaluated carefully and the most reliable ones were selected for the Gibbs energy optimization process. The results calculated by the optimized thermodynamic database showed good agreement with the selected experimental data, particularly on the zirconia-rich side of the system.