The effect of an external electric field on the rate of diffusion-controlled ion recombination was investigated. The Brownian motion of positive and negative ions under the influence of the mutual Coulomb attractive potential and an external electric field was described by using the diffusion equation. Both fully and partially diffusion-controlled recombinations were considered. Analytical expressions for the rate constant were obtained for low electric fields. For high electric fields, the rate constant was calculated numerically. For fully diffusion-controlled recombination, the rate constant always increases with increasing field strength. However, for partially diffusion-controlled recombination, the rate constant decreases with increasing field strength if the intrinsic rate at the encounter distance is very slow. Possible physical mechanisms were proposed to explain these results. Finally, these results were compared with available experimental data on the electric field effect on electron-ion recombination.