The parathyroid carcinoma is rare disorder and its reported incidence is 1-2% of primary hyperparathyroidism. Parathyroid cancer shows more profound clinical symptoms and signs than adenoma or hyperplasia: more severe renal and bony complications, higher calcium levels (>15 mg/dL) and palpable neck mass. It usually diagnosed after operation when it shows malignant pathologic findings, local invasion, distant metastasis, or even recurrence. The treatment is surgical resection. Hypokalemia is relatively common feature of primary hyperparathyroidism but it should be marked because of its arrhythmogenic effect when accompanies with hypercalcemia. We experienced a hyperfunctioning parathyroid carcinoma in 53 year old rnan confirmed postoperatively which showed hypokalemia normalized after operation.