The aim of this study was to investigate the changes of serum malondialdehyde levels in sheep naturally infected with Dicrocoelium dendriticum. Serum malondialdehyde activity was measured in 20 sheep that were naturally infected with D.dendriticum. These sheep were selected to examination of bile ducts and gall bladder after slaughtering. Scores were obtained for the positives and 17 D.dendriticum negative healthy controls. The difference between malondialdehyde levels of sheep infected with D.dendriticum and control group was statistically significant (P<0.001). In conclusion that malondialdehyde levels clearly increase in sheep infected with D.dendriticum.