Agenesis of a hepatic lobe is an extremely rare congenital anomaly and only one dog have been reported in veterinary literature. We encountered a dog with this anomaly diagnosed by Computed tomography (CT) and portography. A two-year-old, 6.9-kg female Shih tzu dog was presented with vomiting and anorexia. The dog had no history of abdominal surgery or trauma. Biochemical analysis showed elevated plasmatic liver enzymes. CT revealed the absence of the liver parenchyma and vascular system of the left lobe, quadrate lobe and papillary process of the caudate lobe. A portosystemic shunt was also observed. The liver parenchyma and vascular system of these lobes were not detected under digital subtraction angiography during laparotomy. Furthermore, the liver parenchyma and vascular system of these lobes were not detected even when the remaining liver volume increased two months after treating the shunt vessel. CT proved itself a good option for antemortally diagnosis of hepatic agenesis in a dog.