A polymorphism of κ-casein gene CSN3 exon IV was identified in Chinese Maiwa Yak by PCR-restriction fragment length polymorphism protocol to reveal the distribution of diversity of κ-casein in Maiwa Yak at the DNA sequence level.The sequences corresponding to the coding region of κ-casein gene exon IV were used for phylogenetic analysis to establish the phylogenetic relationship of CSN3 variants in the Bos genus.Experimental results indicated that most yak milks were with κ-casein BB genotype.A new variant was identified in Maiwa yak breed by analysis and entered into GenBank with accession number GU441771.The phylogenetic analysis presented κ-casein evolution was consistent with species at a degree.