Objective To study the ultrastructure of the anterior cruciate ligament.Methods Specimens were procured from seven patients with arthroscopy and observed under scanning and transmission electron microscope. Results Fibrils of normal anterior cruciate ligament arrayed in compact parallel form with waviness.Connecting fibers existed between fibrils.Diameters of fibrils were different with range of 30nm to 60nm mostly.Fibroblasts were distributed along with the fibrils.Active cells excrete filamentous collagen fibrils.Conclusion The ultrastructure of the anterior cruciate ligament was correlated with its function status.Mature fibroblasts existed in the ligament.