A two-dimensional direct numerical simulation is applied to spray jet flames, and effects of background pressure and droplet size on soot formation behavior is investigated in detail in the mono-dispersed droplet size conditions. N-decane (C₁₀H₂₂) is used as a liquid fuel, and droplet motion is calculated by the Lagrangian method. The extended flamelet/progress-variable approach is emplyed for the turbulent combustion chemistry. A kinetically based soot formation model with the flamelet model is employed to predict soot formation. The results show that the soot is mainly formed in the large vortex structure in the downstream region and the soot formation behavior is significantly affected by the pressure. It is found that the soot volume fraction and soot particle diameter increases with increasing the pressure due to that the concentration of soot precursor increases and the soot particle's residence time is enlongated in the structure.