Combustion characteristics are significantly affected by ambient pressure, which is usually much higher than the atmosphere in the engine cylinder. In this work, the effects of pressure on the flame structure and soot behavior in a methane diffusion flame doped with n-heptane were numerically investigated by a detailed chemical mechanism and a sectional soot model. The results show that the high-temperature region moves toward the wings of flame and the radius of the flame decreases as ambient pressure increases. Soot volume fraction increases significantly with its peak value scaled with p(2.25) for the pure methane flame and p(1.60) for the methane flame doped with n-heptane. In addition, the height at which the initial soot forms moves upstream, while the height at which soot particles are completely oxidized moves downstream, resulting in a larger sooting region. The increase of soot concentration is mainly due to the increase of the mixture density as pressure changes, and therefore, the collision frequency between gas species and particles increases, which in turn accelerates the inception rate and surface growth rate of particles. Primary number density also increases because of the increase in the inception rate. The aggregate characteristics, which are evaluated by the number of primary particles per aggregate, increase because of the increase in the particle number density that leads to an acceleration in particle coagulation. Soot mass addition, dominated by hydrogen-abstraction-carbon-addition reactions, increases because of the increase of absolute concentrations of C2H2 and H radicals, while soot mass consumption, dominated by OH oxidation, increases with a lower rate than that of soot addition because of the increase of the collision frequency and efficiency between the OH radicals and particles.