High fat diet consumption induces fat accumulation in the liver. An understanding of when liver fat accumulation begins is important for exploring the mechanisms underlying fatty liver. The aim of this study was to investigate the processes of fat accumulation in the liver during high fat diet consumption with or without exercise using computed tomography (CT). Male 6 week old C57BL/6J mice were randomly assigned to the normal diet or high fat diet group. After 6 weeks, mice in the high-fat diet group were further divided into voluntary wheel exercise (HFD+Ex) and no exercise (HFD) groups. We measured body weight, food intake and locomotor activity in all mice. Liver fat accumulation was measured by CT scan weekly. Blood and tissue sampling was performed at the end of the experimental period. Following the 6 week exercise period, total body, mesenteric fat and liver weight in the HFD+Ex group were significantly lower than those in the HFD group. Alanine aminotransferase levels improved in HFD+Ex mice compared with those of HFD mice. The hounsfield unit value in HFD mice decreased between 3 and 8 weeks, suggesting that liver fat accumulation accelerated during this period. In contrast this decrease was not observed one week after exercise in HFD+Ex mice. These results suggest that liver fat accumulation estimated by CT was not observed until the 3rd week of high fat feeding while the effects of voluntary wheel exercise appeared immediately.