The effect of pressure on soot formation in a laminar ethylene/air coflow diffusion flame was investigated by experiments and numerical simulation at pressures from 1 to 8 atm. Soot volume fraction was measured by the diffuse-light two-dimensional line-of-sight attenuation optical diagnostic method and calculated by moments model and a relatively detailed gas phase chemistry. The numerical model successfully captured the variation of soot volume fraction with increasing pressure. The detailed analysis of numerical simulation results suggests that although the rates of all soot formation sub-processes increase with increasing pressure, the rates of increase of these sub-processes differ, with that of PAH condensation being the fastest, followed by those of inception and acetylene addition, respectively. At atmosphere pressure, acetylene addition contributes most to soot formation in terms of the overall formed soot mass, while the contribution of PAH condensation significantly increases at high pressures. The variation in the soot formation mechanism is due to the different increase rates of the mole concentrations and formation of several key species that significantly affect soot inception, acetylene addition and PAH condensation. ; peer reviewed: yes ; NRC Pub: yes