Planar laser induced fluorescence (PLIF) technique was established to study the liquid-liquid turbulent micromixing process. A typical process of liquid-liquid cross-flow mixing in flow channels at millimeter scale was visualized by using this non-invasive measurement technique.Transient measurements of concentration field highlighted by the excited fluorescence dye were recorded. A wave-like vortex structure at the scale of 1 to 2 millimeter was found after the two liquid cross flows met each other. The results showed that the development of the vortex was the main factor affecting the mixing performance.The mixing process was also analyzed quantitatively by using the intensity of segregation (IOS).The trend of IOS value vs y locations was also illustrated and the effect of velocity ratio of two liquid flows and Reynolds number of the mixing liquid flow on the mixing process was studied.