Juvenile idiopathic arthritis (JIA) is the most common chronic rheumatic disease in childhood and an important cause of acquired disability in children. In the recent years, musculoskeletal ultrasound (MSUS) has been regarded as a reliable method to precisely document and monitor the synovial inflammation process in patients with JIA. Objectives: To evaluate the role of (MSUS) in detection and assessment of clinical and subclinical synovitis in patients with JIA. Patients and Methods: This prospective cohort study was conducted in the Pediatrics and Physical medicine, Rheumatology and Rehabilitation Departments, Tanta University Hospital. The study included 20 children aged< 16 years diagnosed with JIA according to International League of Associations for Rheumatology (ILAR) criteria. Patients were assessed by MSUS and JADAS 10 at the initial diagnosis and reassessment 6 months after the start of treatment. All the studied patients were subjected to essential laboratory investigations initially and after 6 months of starting treatment. Results: There was statistically significant value between MSUS findings and Patient grade according to JADAS 10 score. MSUS proved to have a significant role in detection of clinical and subclinical synovitis in JIA and a significant role for reassessment of patients to evaluate the efficacy of treatment. JIA proved to be more common in females. Arthritis and Arthralgia were the most common JIA manifestations in our study. Polyarticular RF- JIA was the most common type in our study. Conclusion: MSUS has a great role in detection and assessment of clinical and subclinical synovitis in JIA, so it is used in early detection of JIA patients and used for assessment of disease activity in JIA patients after starting treatment.