In this work, single-port, cylindrical grain laboratory-scale hybrid rocket motors are numerically simulated to study the effect of diaphragms. The effect of single and multiple diaphragms is investigated by varying diaphragm height, its axial location (for a single diaphragm), and spacing (for multiple diaphragms) at selected inlet GOX. Asingle diaphragm increases the local regression rate and its influence is prominent only in the region immediate downstream. Therefore, a tangible increment in average regression rate with a single diaphragm can only be realized for motors with small L/D(< 10). The combustion efficiency in the presence of a single diaphragm may increase or decrease relative to the case with no diaphragm depending on the height and the position of the diaphragm. A diaphragm positioned near the nozzle end of the motor is most effective in increasing combustion efficiency. For long motors (L/D > 10), use of multiple diaphragms are required to increase the regression rate and combustion efficiency. This requires diaphragms to be strategically spaced. Simulations show that regression rate and combustion efficiency improvements are maximum when diaphragms are spaced 8-10 times their height. The results also show that, at optimal spacing, a few tall diaphragms are more effective in improving regression rate compared with many short diaphragms. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.