Since the rectus femoris muscle is associated with trauma and disorders such as muscle strain, it is often a target for evaluation and treatment. However, in many studies, measurement results were obtained from only a part of the rectus femoris muscle and used as a representative value without considering the differences across the muscle. The rectus femoris muscle may change shape with knee flexion because the structure is complicated; it has an intramuscular tendon. The purpose of this study was to assess the changes in shape of the rectus femoris muscle during flexion of the knee joint in different directions. Twelve lower limbs of 12 male university students were analyzed. The rectus femoris muscle was divided into eight parts, and short-axis images were taken with an ultrasonic diagnostic imaging device at the knee joint; in extension; flexion at 30 °, 60 °, 90 °, and 120 °; muscle thickness; muscle width; and cross-sectional area. It was suggested that the thickness of the rectus femoris muscle increased from “A” to “F” due to knee flexion, and that this increase occurred because of stretching at the same site. In “G,” there was no difference between the angle conditions; conversely, in “H,” the muscle thickness decreased due to knee flexion. It should also be noted that D and E have the greatest muscle thickness when measuring in the knee flexion position.