Based on the kinetic equation of particles, an approximate expression is proposed for the coagulation coefficient of bipolarly charged particles with the effect of an alternating-current (AC) electric field. When the external electric field is high enough, the proposed solution agrees well with the previous numerical results of Koizumi, Kawamura, Tochikubo and Watanabe [2000. Estimation of the coagulation coefficient of bipolar-charged aerosol particles. Journal of Electrostatics, 48, 93-101.]. (C) 2008 Elsevier Ltd. All rights reserved.