Abstract: This paper studies the flow process of two stratified fluids in a cylinder, using the Planar Laser Induced Fluorescence and the High Speed Camera techniques, covering both miscible and immiscible stratified liquids. It is shown that the directions of the liquid density gradient and the viscosity gradient are the key factors for the interface instability between liquids. When the directions of the two gradients are the same, the violent mixing would not occur during the spin-up process, and the light fluid will impact the heavy fluid during the spin-down process. When the directions of the two gradients are not the same, the pumping action occurs during the spin-up process, the interface breaks up in the later period of the spin-up, and the heavy fluid will impact the light fluid during the spin-down process. Among the three stages of spinning-the spin-up, the rigid body rotation and the spin-down, the spin-down has a big effect on the mixing process, which means that no matter whether the directions of the two gradients are the same, the interface instability between liquids will occur.