Matched pressure injection through diamond-shaped injectors was applied to a Mach 2.5 supersonic cross-flow, and penetration and mixing characteristics of the injected plume were experimentally investigated. Both subsonic and supersonic injections were investigated at various injection pressure. The subsonic injection case was utilized to evaluate effective back-pressure to the plume, which was almost the same value to that on a solid wedge surface with identical wedge angle to the injector orifice at designed flow rate for matched pressure condition. With the supersonic injection, the plume floated from the injection wall, and best penetration height was attained. The penetration height increased at over-expanded condition, while maximum mass fraction decay was insensitive to the injection condition. With the subsonic injection, the plume shaped like a pillar, leaving certain fraction of the injectant within the boundary layer region. The penetration height as well as the maximum mass fraction decay was found to be insensitive to the injection pressure.