Insulin-like growth factor-I (IGF-I) is one of the most important metabolic factors that plays a critical role in cow fertility. Recently, point mutation of IGF-I gene at nucleotide position 512 (C/T transition) within the 5′-untranslated region (5'-UTR) has attracted the most attention. It has previously been demonstrated that there was an important relationship between this type of mutation and IGF-I concentration in dairy cows serum. Hence, we investigate the relationship between polymorphism within 512 of the 5'-UTR of IGF-I gene and some reproductive parameters in Iranian Holstein dairy cows. Blood samples were collected from 60 multiparous cows with a history of desirable fertility (SPC<2; n=25) and undesirable fertility (SPC2; n=35) based on the previous lactation numbers. The results showed that higher percentage (65%) of cows with CC genotype had desirable fertility (SPC<2); while a lower percentage of cows with TC (33.3%) and TT genotypes (14.3%) had good fertility (P<0.05). Therefore, the CC genotype was associated (P<0.05) with desirable fertility. In addition, the IGF-I mutation had a significant effect on service per conception and days open in which cows with TT genotype and TC genotype had greater average of services per conception and days open compared to cows with the CC genotypes (P<0.05). It can be concluded that the C/T mutation within this region of IGF-I gene may influence the reproductive parameters in Holstein dairy cows. These data support the hypothesis that the mutation in this site might be considered as a genetic marker for reproductive performance in Holstein dairy cows.