This chapter is dedicated to investigating the fluid flow and heat and mass transfer of homogeneous condensation of water vapor in a supersonic nozzle due to the complex interaction of the supersonic flow, phase transition and shock waves. Non-equilibrium condensation model for both moist-air and pure steam flows through nozzles was built. The effect of the inlet subcooling and supersaturation on the condensation behavior is conducted to evaluate the performance of the supersonic separation focusing on the shock wave interaction. It is extremely expected to obtain an algebraic expression about the nucleation process, especially the flow properties at the Wilson point. An analytical solution for predicting the position of Wilson point and local thermophysical properties in nozzle flow at low pressure was proposed according to the classical nucleation and the droplet growth models joining with Lagrangian momentum, energy and Clausius–Clapeyron equations, simultaneously considering the expansion rate coefficient derived by Rayleigh line relationships. The results showed the analytical solution agrees well with the numerical results and the experimental data.