K-Casein (CSN3) is a phosphoglycoprotein that constitutes approximately 12% of the casein complex of bovine milk. The amount and type of CSN3 present in milk vary considerably depending on the individual cow. The use of small quantities of 2-mercaptoethanol in electrophoretic separation led three independent groups to the discovery of two distinct bands of protein denoted as A and B corresponding to CSN3. Later studies showed that the presence of these bands were due to genetic polymophism of this protein. Variant B differs from A by having isoleucine and alanine at positions 136 and 148, respectively, instead of threonine and aspartic acid in the 169 amino acid sequence. In the most cattle populations, there is a predominance of the A over the B type of CSN3 (Ng-Kwai-Hang et al. 1991). However, it has been demonstrated that CSN3 B would be more desirable than CSN3 A because this form is associated with higher casein content in milk, higher cheese-yielding capacity, more favorable cheese composition and better coagulating properties in terms of rennet clotting time and curd firmness (Ikonen et al. 1999). Therefore, it seems desirable to increase the frequencies of B type of CSN3 through appropriate selection and breeding schemes. This paper reports the genotyping for CSN3 gene and his allele frequencies done for 53 animals of Mongolian breed. Because, heretofore genotyping of CSN3 gene for this breed has not reported, moreover milk of this animals used in cheese industrial.