Milk production and composition are affected by many different factors including genetic and environment. In the physiology part, growth and differentiation are result of gestation and milking and causes to mammary gland secretion. Peak of milking period and its duration are in point of selection. In dairy cows breeding with genetic selection that is enhancer of milking peak and persistency, we can increase frequency of favorite alleles that affect selected traits. Candidates in somatotropic axis have an important role in this way. These candidates have polymorphisms in the part of their gene that determine genotype of this polymorphism which can be used in the genetic selection. One of these candidates is IGF-I that is very important in growth, differentiation and also in apoptosis and involution of mammary gland cells and it can also act on milk production and compositions indirectly. The objective of this study was to determine IGF-I polymorphism gene and its relation with Holstein dairy cow’s performance. For the experiment, blood was collected from 250 Holstein cows of four different herd managements in Isfahan province. Genomic DNA was extracted from whole blood with salting out procedure. Genotypes of IGF-I were identified with PCR-RFLP technique. The polymerase chain reaction was used to amplify the 249 bp DNA fragments from genomic DNA. PCR products were digested with Sna BI enzyme. In this reaction with the presence of T nucleotide in the regulatory region of the gene, 512 bp upstream of the initiation codon, 249 bp DNA fragments convert to 226, and 23 bp DNA fragments and it named A allele and with point mutation T to C nucleotide, this fragment remain without change. Reaction digest productions were distinguished on polyacrylamide gel and AA, AB and BB genotypes were recognized and then their frequencies determined. B allele with 0.542 and AB genotype with 0.630 had most frequencies in this study. After that with GLM procedure of the SAS program, genotype and herd effects as fix effects and milking days, open days and dry days as covariates, we determined the relation between genotype and production (raw milk, twice mature 305 days milk) and composition (raw fat and protein, percentage of fat and protein and twice mature 305 days fat and protein) milk, reproduction (gestation period days, calving interval and calving interval to first insemination) and birth weight traits. Through of these traits only percentage of fat and protein had significant relation with genotype (p 0 1 and p 0.05 respectively). AB genotype had more effect than other ones. Thus if the purpose of selection is increase of fat and protein percentage, we can have deserve selection with this information.