The temporal change in fluid mixing patterns often appears to differ from the motion of fluid matter, especially in the case of unsteady flow. Fluid mixing patterns essentially concern the motion of a body comprising an infinite number of fluid particles. It cannot, therefore, be reduced to the motion of finite number of fluid particles. With the passage of time, the mixing pattern tends to take on a common pattern independent of the initial one. This fact strongly suggests the existence of a latent flow field for the pattern generation intrinsic to the fluid flow. However, traditional fluid dynamics lacks a framework to explain these problems appropriately. In this article, the new concept of velocity vector field is introduced which directly controls the motion of a fluid mixing pattern. With this concept, the phenomena of fluid mixing are grasped from a different point of view, and the essence of these phenomena is proposed more clearly.