Adrenocortical oncocytomas have rarely been reported on in the medical literature, and most of them have been nonfunctional and benign. We report here on a case of a 43-year-old man with a left abdominal mass. The patient showed no signs of hypertension or hormonal imbalance. The abdominal CT scans showed a huge mass that measured 11 cm in diameter, and it was located at the left adrenal area. Grossly, the tumor was well encapsulated and homogenous with central necrosis. Microscopically, the tumor was composed of oncocytes with abundant granular cytoplasm. Immunohistochemically, these cells were diffusely positive for cytokeratin and focally positive for synaptophysin and NSE. The ultrastructural studies showed numerous mitochondria in the cytoplasm. We will discuss the criteria that indicates malignancy as presented by Weiss et al. and we summarize the difference between conventional and oncocytic adrenocortical neoplasm. This case showed some features of malignancy based on the criteria presented by Weiss et al.