The effect of relative humidity on the electron distribution and the ozone production in the direct current (dc) corona discharge from a thin wire is evaluated with a numerical model. The model is based on the prior models of ozone production by dc coronas in dry air, with modifications to incorporate the effect of water vapor on the corona plasma and the plasma chemistry for ozone production. The distribution of electron number density is obtained by solving coupled continuity equations for charge carriers and the Maxwell's equation. The electron kinetic energy distribution is solved from the Boltzmann equation. Both the electron number density distribution and the distribution of electron kinetic energy are insensitive to the variation in the relative humidity due to the relatively low water content. The electron distribution data are then combined with the chemical mechanisms of ozone production in humid air and the two-dimensional (2-D) transport processes to obtain the distribution and the production of ozone. In qualitative agreement with prior experimental findings, the ozone production rate decreases with increasing relative humidity in both positive and negative coronas due to the removal of atomic oxygen by water molecules, and the ozone production rate in the negative corona is one order of magnitude higher than that in the positive corona.