Flow mixing in the recirculation zone of a backward-facing step flow (free stream velocity, U-o = 10 m/s, and step height, H = 20 mm) with gas injection is visualized by laser sheet imaging, based on the Mie-scattering concept. Two types of imaging have been done. Instantaneous visualization was done by using an Nd-Yag pulse laser. The continuous visualization, on the other hand, was done by using argon ion laser and a high-speed video camera to discuss the continuous motion of mixing. The study focuses on the effects of the specific momentum ratio (I = 0.04, 0.1 and 0.3) and the location of injection of nitrogen gas from a two-dimensional slot port (l(r)/H = 2 and 4) on the dynamic nature of the injected gas trajectory and gas distribution in the recirculation zone. The result shows that with higher specific momentum ratio, in case of the injection at near step (ICH = 2), the injected gas distributes more toward upstream since the fluid dynamics restriction is less due to a lower recirculating now velocity. Meanwhile, in case of injection near the reattachment point (l(r)/H = 4), the injected jet, which penetrates the flow field deeply, is deflected by the free stream so that the gas distributes more toward the downstream region. In both cases, the mean normalized luminosity in the region where the gas distributes more is no less than 1.6 times that in the region where it distributes less. Mixing is more rapid and intense in case of injection near the reattachment point due to higher turbulence. Furthermore, two frequency modes in the motion of the injected gas jet can be observed. The tower frequency mode associated with the shortening-lengthening of the recirculation zone (flapping) is not affected, whilst the higher one associated with the interaction of vortical motion of the now field and the injected jet now is influenced by the specific momentum ratio of injection.