Objective：To investigate the clinical significance of plasma free metanephrines（MNs） in the diagnosis and prognosis of pheochromocytoma by high performance liquid chromatography-electrochemical detection（HPLC-ED）. Methods：HPLC-ED was carried out to detect plasma free MNs，including 21 patients with pheochromocytoma，26 healthy volunteers，23 patients with primary hypertension and 24 patients with adrenal mass but without pheochromocytoma. Results：Plasma concentrations of free MNs in patients with pheochromocytoma were significantly increased compared with those of other groups（P<0.05）. Taking normetanephrine（NMN） 150 ng/L and（or） metanephrine（MN） 120 ng/L as cut-off value，the sensitivity and specificity for diagnosis of pheochromocytoma were 100% and 89% respectively. Plasma concentrations of free MNs showed significant positive relationships with tumors size in patients with pheochromocytomas（NMN：r=0.952，P=0.000；MN：r=0.510，P=0.018）. Plasma concentrations of free MNs in patients with pheochromocytoma dropped rapidly after the surgery. Conclusion：Plasma free MNs are good markers for the diagnosis and progno－sis of pheochromocytoma.