Supersonic injections through diamond-shaped injectors were applied to a supersonic combustor with a diverging section, with a Mach 2.4 incoming vitiated airflow at a total temperature of 2000 K, and combustion characteristics were experimentally investigated and compared to those with sonic injection through circular injectors. Without any flame-holding devices installed, scramjet-mode combustion was attained in wide range of fuel flow rate, in which the supersonic injections resulted in a higher pressure-rise in far-field than the sonic injections. Regardless of the injection scheme (i.e, orifice shape, orifice number, and injection conditions), transition to dual-mode combustion enhanced the thrust production, and the supersonic injection through the diamond-shaped orifice had lesser ability to cause the transition than the sonic injection through the circular orifice. The ability was enhanced by installation of a recess and introduction of recess fueling, and resulting combustor performance was slightly better than that with the sonic injection, in the case with single orifice. Both the number of the orifice and the injection pressure (by changing the total orifice throat area) were varied for the supersonic injection through the diamond-shaped orifice for fixed fuel flow rates, and the change in combustion characteristics were also reported.