The unsteady flamelet model is applied in a numerical simulation of soot formation in a turbulent C2H4 jet diffusion flame. A kinetically based soot model is used, which relies on a detailed kinetic mechanism to describe the formation of small polycyclic aromatic hydrocarbons. To describe the formation, growth, and oxidation of soot particles, flamelet equations for the statistical moments of the particle size distribution are derived. Since the effective Lewis number of large particles tends to infinity, a formulation is given, which allows the investigation of the effect of different diffusion coefficients of the particles on the soot formation process. The results of the calculation are compared to experimental data, showing very good agreement for the temperature, which is shown to depend strongly on soot and gas radiation. The predicted soot volume fraction compares reasonably well with the measured data, if differential diffusion of the particles is considered. Calculations with unity particle Lewis numbers show similar results, but overpredicts the soot volume fraction in the rich part of the flame.