Spatiotemporal databases have received increasing interests, and a lot of data models have been proposed. One of the key issues in spatiotemporal databases is the representation of spatiotemporal changes. The typical example is to represent moving regions. But most of previous work concentrated on representing discretely moving regions. Continuously moving regions are much difficult to be modeled, and few techniques are proposed. This paper deals with the representations of continuously moving regions. The new approach is called constraint rectangle. A continuously moving region is represented as a set of constraint rectangles, and both x-dimension and y-dimension of each constraint rectangle are functions of time. We develop algorithms to represent a moving region into constraint rectangles, and discuss the prototype implementations.