Objective: The purpose of this study was to investigate changes of abdominal muscles thickness according to the angle during the active straight leg raise (ASLR) in young healthy subjects. Design: Cross sectional study. Methods: Twenty-three healthy university students (13 men and 10 women) voluntary participated to the study in S University. The ASLR was performed with the subject lying supine with lower extremities straight on a standard plinth, hands resting on the chest, and elbows on the plinth. When one subject performed ASLR from each angles (30°, 45°, 60°, 90°), compared changes in the thickness of rectus abdominis muscle. Changes in muscle thickness during ASLR test were assessed with ultrasonography. All subjects were to provide enough time of rest after performed ASLR. Rectus abdominis thickness were measured using rehabilitative ultrasound image. Results: Good quality rectus abdominal muscle activation data were recorded during ASLR. The length changes of linea alba showed significantly shorter in between 0° and 30° (p＜0.05). The thickness of rectus abdominis muscle were significantly different between 0° and 30°, 0° and 45°, 0° and 60°, 0° and 90°. According to increase of pelvic angle, the thickness of rectus abdominis muscle were more thickening (p＜0.05). Conclusions: This result is changes of abdominal muscles thickness according to the angle during the ASLR.