Plasma sheath theory is applied to understand the plasma behavior in electric field modified flames. This paper presents a set of 1D plasma sheath equations with approximated analytical solutions to calculate the sheath thickness for given applied voltages and plasma properties. The results show that the anode sheath is ten of microns thick, less than 1 V, and largely independent of the applied voltage. The cathode sheath grows with the applied voltage to centimeters thick. The limited extent of the anode and cathode sheaths, which limits the reach of the electric field, in part explains the different flame behaviors reported in the literature. The ionic wind body force is also calculated based on ion energy losses due to collisions. The sheath analysis provides a possible explanation for reported flame behavior under a DC field modified such as saturation current and diode-like behavior. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.