In this paper a novel algorithm for motion estimation of the endometrium in a sequence of ultrasonic images is presented. The algorithm used is based on the criterion that a pixel of a motion object at different times has a different gray value. The motion estimation includes motion frequency and thickness; the former one is determined by the detection of change of a pixel value in a sequence of images during a given time period, and the latter is the result of the object's dimension divided by its extended width. The algorithm has been tested on synthetic and real images, and the results are encouraging.