A zero flux boundary condition is presented for the particle turbulent mixing diffusion equation in a wire-plate electrostatic precipitator. Since the wire-plate electrostatic precipitator is symmetric along the wire plane, only half of the symmetric plane is analyzed. This paper presents the zero flux boundary condition for the turbulent mixing diffusion model at the wire plane of a wire-plate electrostatic precipitator, partial derivativeC/partial derivativey = 0 at y = 0 instead of the traditional form which is E-y(partial derivativeC/partial derivativey) - wC = 0. The improved boundary condition is validated by comparing the theoretical results with experimental data [Salcedo & Munz (1978), Proceedings of the Third International Conference on Electrostatic Precipitation, Abano-Padova, Italy, pp. 343-359]. The zero flux boundary condition provides better results than that of the traditional form. (C) 2002 Elsevier Science Ltd. All rights reserved.