The combustion characteristics obtained from the coupling effect between swirl injection and a novel composite hybrid rocket fuel grain were investigated. The novel composite fuel grain was composed of a three-dimensional printed acrylonitrile-butadiene-styrene (ABS) helical substrate with an embedded paraffin-based fuel. Two swirl flow injectors with opposite injection directions were designed: one with the same injection direction as the swirling direction induced by the ABS helical substrate and the other having the opposite direction. Axial injection trials were also performed as a baseline for comparison. Firing tests were carried out using a lab-scale hybrid rocket engine with oxygen as the oxidizer at average mass flux values in the range of 2.1-4.5 g/ (s.cm(2)). When combined with either swirl flow injector, the novel composite fuel grain exhibited excellent combustion properties. Compared with the axial flow injector, both swirl injectors greatly improved the regression rate of the novel composite fuel grain, by 82.4% and 50.6% respectively, at an oxygen mass flux of approximately 4.25 g/(s.cm(2)). Three-dimensional imaging of the inner surface of the novel composite fuel grain and cold flow numerical simulations were employed to assess the mechanism by which the regression rate was increased. The results of these analyses elucidated the effects of different injection methods on the oxidizer-tofuel ratio distribution during combustion of the novel composite fuel grain.