Abstract: The objective of this study was to obtain genetic marker information in the Iranian Holstein bulls by analyzing bGH gene polymorphism and to verify its association with milk production traits. One sample including 134 Iranian Holstein bulls was genotyped at bGH gene restriction sites (bGH-AluI). Gene frequencies of L and V alleles were 0.936 and 0.064, respectively. Genotypes are distributed according to the Hardy-Wienberg equilibrium. Differences in the milk and fat yield between the two genotypes approached significance (p≤0.10). Bulls with LL genotype had higher milk, fat yield compared to LV genotypes. Present study showed that the L allele was associated with protein yield (p≤0.021). No associations (p>0.1) were found between the genotypes and fat and protein percent. The dominant L allele at the bGH locus favours higher production of milk protein. It can be concluded that this locus is a QTL or is in disequilibrium linkage with one or more tightly linked QTL. Thus it could be a useful DNA marker for milk production traits in dairy cattle breeding.