Objective To investigate the diagnostic value of two-dimensional and color Doppler ultrasonngraphy on diagnosis and judgement of prednisone withdraw in patients with subacute thyroiditis.Methods Sixty-six patients with clinically proved subacute thyroiditis underwent two-dimensional and color Doppler ultrasonography before treatment of prednisone. The ultmsonographic features and bloodtlow distribution in thyroid of these patients were analyzed retrospectively. Target of prednisone withdraw was judged according to ultrasonographic images (36 eases) and clinical symptom (30 cases). Results The ultmsonographic features of subacute thyroiditis mainly presented as bilateral or unilateral diffussion or focal low echogenicity in the affected thyroid, and color Doppler ultrasonography showed the increased vascularity around the lesions. Intraarterial bloodflow feature was low velocity and low resistance pattern, prednisone withdraw was performed according to ultrasonographic images after the treatment of prednisone, then relapse rate was decreased compared with the group of clinical symptom [8.33% (3/36) vs 26.67% (8/30)] (P <0.05). Conclusion Two-dimensional and color Doppler ultrasonography is highly valuable for diagnosing and treating subacute thyroiditis.