In this work, the effect of solid oxidizer (AP) mixed fuel grain (HTPB) on regression rate and performance in GOX hybrid rocket motors is numerically studied. Here the pyrolysis products of AP and the fuel forms a premixed flame very close to the grain surface in addition to the fuel–oxygen diffusion flame located further away from the grain surface. The presence of premixed flame increases heat feedback to the grain and consequently increases the local regression rate throughout the grain length. This is contrary to the locally limited influence observed while using other regression rate enhancement techniques like swirling oxidizer, or transverse diaphragms/protrusions. Scaling of motors with AP mixed grain of a given composition follows that of a pure fuel grain where the grain geometry and the inlet oxidizer mass flux need to be maintained.