BACKGROUND/AIMS: In spite of increasing interests about nonalcoholic steatohepatitis (NASH), there are few reports about the ultrastructure of hepatocyte in this disease. The aim of this study was to clarify abnormal electron microscopic (EM) findings and related factors in NASH. METHODS: Total of fourteen patients who underwent liver biopsy due to steatohepatitis were included. Precise personal history was taken and variable blood tests such as liver function test, lipid profile, and serum iron study were done. Pathologic examination with light and electron microscopy was done by single pathologist. RESULTS: Eleven men and three women were included and mean age was 33.7+/-12.8 years. Nine patients drinking less than 40 g/week was grouped as "NASH group" and other 5 patients drinking more than 40 g/week and body mass index less than 25 was grouped as "ASH (Alcoholic Steatohepatitis) group". Polymorphism of mitochondria such as megamitochondria or loss of cristae was major abnormal EM findings and was more common in "NASH group" than "ASH group" (p=0.027). There was no significant clinical or pathological factors related with the presence of these abnormal EM findings. CONCLUSIONS: Polymorphism of mitochondria is major abnormal EM finding of steatohepatitis and is more common in NASH than ASH. And there is no significant clinical or pathological factors which could predict the presence of these abnormal EM findings.