Background：Patellofemoral pain syndrome（PFPS） is a common knee disorder characterized by anterior or retropatellar pain associated with activities that load the patellofemoral joint. Previous studies reveal that the vastus medialis obliquus（VMO） is an important dynamic medial stabilizer of the patellar. Insufficiency of the VMO leads to lateral shift of the patella and the increases the patellofemoral contact force. An in vitro study conducted by Hubbard JK. et al. claimed that there were no significant relationship between several morphologic characteristics of the VMO and the extent of patellofemoral joint deterioration. We consider that the condition might be different in vivo, so we chose ultrasonogrphy as the measurement tool to examine the morphology of the VMO in PFPS patients and healthy controls. Purpose：To determine if there are significant differences in several morphologic parameters of the VMO between patients with PFPS and healthy controls under 50. Method：31 PFPS patients and 31 matched healthy adults under 50 were recruited for the study. The HDI 5000 ultrasonography machine was used to evaluate morphologic parameters of the VMO, including the percent of patella attachment, fiber angle, the volume attached to the patella, and the change of shape of the VMO.