An undisturbed re-organisation of the cervix postpartum is a basic pre-condition for an undisturbed puerperium (Wehrend et al., 2003). This aspect of the puerperal involution has been insufficiently considered in the literature. It was shown that some cows developed a disorder of the cervical involution (Wehrend and Bostedt, 2003). LeBlank et al. (2002) concluded that the size of cervix in cows with clinical endometritis were associated with a decrease in pregnancy rate. With the use of the ultrasonic technique instead of using fingers of the human hand, more exact data on cervix regression can be delivered. Meanwhile, little is known about the effect of diet on cervix regression after parturition. The objective of this study was to evaluate the effect of palm oil fatty acids on cervix regression during the first 40 days postpartum in Iranian Holstein dairy cows using ultrasonic technique.