Abstract Background: Juvenile Idiopathic Arthritis (JIA) is defined as arthritis of unknown etiology beginning before the age of 16 years and persisting for at least 6 weeks, while excluding other known conditions. Aim of Study: The purpose of this study is to highlight the beneficial role of MRI & US in the evaluation of knee joint affection in patients with juvenile idiopathic arthritis, especially in early cases. Patients and Methods: The study was carried out on fourty patients (26 females and 14 males), referred to the Radiology Department of Kasr El-Ainy Hospital from Abo El-Rish Pediatric Hospital. Their age ranged from 2.5 years up to 13 years. All patients underwent examination of the more symp-tomatic knee joint using MRI with intravenous contrast (Gadolinium) and Ultrasound (US) examinations. The results of the ultrasound were compared to those of MRI, with the MRI being the gold standard of diagnosis. Results: Among the studied cases ultrasound was able to detect joint effusion in all cases as a compressible anechoic area. It was able to detect synovitis as synovial thickening and increased vascularity on power Doppler in all cases which is evident mainly in the suprapatellar recess. The accuracy of US regarding both effusion and synovitis was 100%. Ultra-sound had great potential to identify the normal cartilage and allows for differentiation of the abnormal morphology such as loss of clarity, irregularity, and defects on the surface. In our study ultrasound was able to demonstrate most of the cases. The overall accuracy regarding cartilage changes was 90%. In this study six of the cases had bone erosions which were detected on MRI. US was able to detect four of these cases. The overall accuracy of US regarding erosions was 95%. Conclusion: Ultrasound has the ability to demonstrate knee joint pathology in early JIA which can help start early treatment or modify already existing one to prevent permanent joint damage. At this point in time, however, it is not possible to determine that ultrasound is superior to MRI, especially regarding bone erosions and the fact that it is operator depend-ent and needs experience.