Abstract: In order to investigate the nutritional composition and volatile substances of yak milk after fermentation and post-fermentation, the changes of nutrients and volatile substances of yak milk, fermented yak milk and post-fermented yak milk were measured, and the nutrition and quality of yak milk were evaluated. The results showed that the lactose content of yak milk after fermentation was significantly decreased (P < 0.05), and the decreased rate was 39.43% . The contents of total, essential, non-essential and delicious amino acids of yak milk after fermentation were significantly increased (P < 0.05), and the increased rate were 64.26%, 57.52%, 69.33% and 56.67%, respectively. The content of fatty acid of yak milk after fermentation has no significantly difference (P > 0.05), but the types and relative contents of alcohols, aldehydes and esters in yak milk after ferentation were greatly decreased. There were no significant differences in lactose, amino acid and fatty acid contents between post-ferentation and ferentation of yak milk (P > 0.05), but the types and relative contents of alcohols, phenols, esters and ketones in post-ferentated yak milk greatly increased compared to fermented yak milk. In summary, fermentation improved the nutritional value of yak milk in some extent, and post-fermentation promoted the accumulation of volatile substances of yak milk. Fermentation and post-fermentation had positive effect on the improvement of nutrition and quality of yak milk.