Objective To analyze X ray characteristics of synovitis of knee joint and suggest a simple diagnostic method. Methods The X ray films of knee joint from lateral view of 30 normal individuals and of 116 patients with synovitis were compared for the features of synovialis. Results Thickening of synovialis and/or edema of infrapatellar fat pad was found in those with knee joint pain of unknown origin, but without abnormal bony changes by radiography. Conclusion X ray film can diagnose synovitis of knee joint, but the etiology needs further investigation.