Objective: To evaluate the utility of magnetic resonance imaging (MRI) in diagnosis of juvenile dermatomyositis and polymyositis (JDM-PM) in children. Method: Fifty-four patients with JDM-PM in the active stage were enrolled in the study group. Twelve patients with benign acute childhood myositis and forty patients with juvenile idiopathic arthritis (JIA) complicated with myositis were enrolled as controls. MRI imaging of thighs was performed in all patients, fast spin echo T1WI, T2WI, and STIR were obtained in all patients.Muscle biopsy was performed in 41/54 patients with JDM-PM. We compared the value of MRI in diagnosis of JDM-PM with muscle biopsy, electromyography and serum aspartate transaminase (AST), alanine transaminase (ALT), creatine kinase (CK), isoenzyme of creatine kinase (CKMB), lactate dehydrogenase (LDH), hydroxybutyrate dehydrogenase (HBDH) levels. Continuous normally distributed variables were reported as means and continuous non-normally distributed variables as median. Chi-square test and Fisher exact test were used to test differences between MRI and other categorical variables. Result: A total of 54 patients were included. Twenty-seven patients were male and the others were female. Average age of the patients was (7.1±3.5) years (2-13 years); 45(83%) paitests were JDM cases and 9(17%) patients had JPM. All patients had MRI examination. Of the 54 patients, 53 had multiple myositis; 10 out of 50 (19%) patients received second MRI after treatment, 6 out of 10 patients had normal findings, 4 patients showed obviously improved images; 41 out of 54 patients underwent muscle biopsy; 22 out of 41 patients had inflammatory cells infiltration and muscle fiber degeneration. The results of the muscle enzyme tests are as follows: 27 (50%) patients had elevated AST, 24 (44%) patients had elevated ALT, 22 (41%) patients had elevated CK, 18(33%) patients had elevated CKMB, and LDH rose in 30 (56%) patients, HBDH rose in 28(52%) patients. These results suggested that muscle MRI was more sensitive than muscle biopsy and muscle enzyme tests in diagnosis of JDM-PM. Conclusion: Patients with JDM-PM showed diffuse patchy hyperintense signals on T2WI of their thighs. MRI may be a sensitive, reliable, and noninvasive tool for clinical diagnosis and theraputic evaluation of JDM-PM.