Aim To explore the sensitivity and specificity of transcranial Doppler to detect mild stenosis of extracranial internal carotid artery. Methods The flow velocity and pulsatility index were measured with transcranial Doppler in 110 patients with mild stenosis (<50%) of extracranial internal carotid artery. Results Transcranial Doppler had sensitivity of 88.3% (mean flow velocity) and 92.6% (systolic peak flow velocity) with specificity of 97.3% to detect the mild stenosis of carotid artery. The length and thickness of atherosclerotic plaque were closely correlated with the flow velocities (r=0.34, p<0.05 and r=0.48,p<0.01). The flow velocity was higher in patients with 21%～49% stenotic area than those with <20% stenotic area, but statistical analysis did not discriminate (p>0.05). Conclusions Transcranial Doppler might be of some significance in determining the atherosclerotic plaque and mild stenosis of carotid artery.