The results of a systematic experimental investigation on the methods of enhancing the regression rate in hydroxyl-terminated polybutadiene (HTPB) fuel used in an HTPB/gaseous oxygen hybrid motor are presented. The effects of the addition of ammonium perchlorate (AP) or aluminum in the fuel, the variation of oxidizer-fuel ratio, and the variation of characteristic dimensions of fuel grain are presented. For the extents of the parametric variations considered, while the addition of AP and/or Al and the reduction of grain port diameter enhance the regression rate, the effect due to the latter is the most significant one. Furthermore, the regression rate increases along the axis, and it becomes essentially constant in the port region corresponding to a fuel-rich composition. The possible physical processes for all of these behaviors are discussed. The experimentally obtained exponents of the variables for regression rate (oxidizer mass flux and port diameter) are found to be significantly different from those of the conventional theory. The similarity between the fuel regression rate equation used in solid-fuel ramjet and that obtained in hybrid motor is discussed.