Because of the DGAT1 K232A polymorphism in the diversification of milk fat has been proven, this study was to assess the frequency of DGAT1 K232A polymorphism in Iranian Holstein dairy cows. In addition, the other aim in this study was estimating the association between the DGAT1 K232A with productive trait performance of milk as well as the somatic cell score of milk (SCS) in this population. The gene diacylglycerol acyltransferase 1 (DGAT1) had been mapped to the centromeric end of the bovin BTA14 and encodes the acyl-CoA: diacylglycerol acyltransferase1 enzyme which catalyses the final step in triglyceride synthesis. Because more than 98 percent milk fat consists of triacylglycerol so DGAT1 enzyme, is a key enzyme in milk triacylglycerol synthesis in the mammary gland. The considered polymorphism results from AA?GC nucleotide substitution in DGAT1 gene exon 8 and so, alanine to lysineof amino acid change at product protein. This polymorphism proved to significantly affect the percent fat in milk. DGAT1 gene was studied in this population (408 cows) by PCR-RFLP technique. A 411 bp fragment including the considered polymorphism was amplified and digested with the enzyme Cfr I to determine the genotype of 408 Holstei cows. Both alleles A and K were observed in population. Three types of genotype including KK, KA and AA were detected after digested by the restrictive enzyme. AA individuals have not mutant alleles, KA individuals have one mutant allele and KK individual have two mutant alleles. Frequency of genetic AA, KA and KK genotypes were respectively 0.3578, 0.5515, and 0.0907 and frequency of K and A allels were estimated 0.37 and 0.63, respectively. For the statistical analysis investigation relationship between DGAT1 K232A with productive traits of milk and somatic cell score of milk in first lactations, the software SAS, procedure GLM was used. In the analysis model the effects of genotype and combined effect of herd, year and season of parturition were considered as fixed effect and days in lactitation, open days and milk production were considered as covariables. The statistical analysis showed significant effects (p 0.05) of the genotype for corrected milk production, daily milk production average, fat percent and fat breeding value traits but genotypes haven’t significant effect for protein percent, milk breeding value and somatic cell score of milk. Estimated average effects of substituting the lysine allele for the alanine variant on record correctted of milk production, daily milk production average, fat percent, protein percent, somatic cell score of milk and o milk breeding value and fat breeding value were -155.56, -1.5, 0.1978, 0.0452, -0.0848, -102.98 and 3.39 respectively. These results can be showed that allele K is effect on increase fat of milk and decrease milk production. Also allele A is effect on increase milk production and decrease fat milk. The relatively lower frequency of the K allele than the A allele may be due to selection for milk yield in recent years. Results showed that the DGAT1 K232A polymorphism can be a source that underlies the reported quantitative trait loci for fat content trait in the proximal region of bovine chromosome 14.