The aim of this study is to investigate soft tissue disorders of affected shoulders after nerve-sparing selective neck dissection (SND) in patients with head and neck cancers (HNCs) by sonography. Eighteen HNCs patients with shoulder disability after SND were enrolled. Shoulder motions, pain, and the sonographic findings were measured and analyzed. Significantly decreased flexion, abduction, and increased pain were found on affected shoulders compared with normal shoulders. There were significant decrease in thickness of trapezius muscle (p=0.001), abnormal findings of supraspinatus tendon (p=0.022), and subdeltoid bursa (p=0.018) on surgical side. The ratio of trapezius muscle atrophy was related to shoulder pain (p=0.010). Patients with subdeltoid abnormalities had significant limitation on shoulder flexion and abduction. Abnormalities of supraspinatus tendon and subdeltoid bursa on sonography and trapezius muscle atrophy may play a key role in shoulder pain and shoulder flexion and abduction limitations.