In the present study, a computational fluid dynamics work was performed to investigate the effects of initial degree of supersaturation at reservoir condition on under-expanded supersonic jet structures, such as Mach disk location and diameter, barrel shock wave and jet boundary locations. The axisymmetric nozzle geometry investigated was a contoured converging nozzle with long straight part. As a result, it was found that the characteristics of flow with occurrence of non-equilibrium condensation in the jet were different from those without condensation. Nomenclature c p specific heat at constant pressure, J/kg°C D diameter, m E,F numerical flux, -£, total energy per unit volume, J/m 3 g condensate mass fraction, -H,Q source term, -/ nucleation rate, 1/m 3 s κ Boltzmann constant, J/K. L latent heat, J/kg Μ molecular weight, kg/(kmol) ρ static pressure, Pa p, r saturation vapour pressure of condensate droplet with radius of r, Pa p St co flat film equilibrium vapour pressure. Pa /?, S viscous term, -Re Reynolds number, -r droplet radius, m r c critical droplet radius, m gas constant, J/(kg K.) Τ temperature, Κ t time, s U conservation mass term, -u, ν Cartesian velocity components, m/s x,y Cartesian coordinates, m γ ratio of specific heats, -ρ density, kg/m 3 σ surface tension, N/m τ shear stress, Pa Subscripts 0 a I m η s ν 00 stagnation point air liquid mixture nozzle exit saturation vapour plane surface