Abstract: Abstract: There are more than 16 million yaks in China, which accounting for about 95% of total amount in the world. The yak beef could be a new resource for meat industry in China with high protein, low fat and unique flavour. However, the commercial potential of yak beef is limited because of the poor tenderness of yak beef. The aging could be a simple and effective method to improve tenderness of beef especially prolonged aging time more than 14 d. In order to investigate the prolonged aging time on quality traits of yak meat, the pH value, tenderness (WBSF, Warner Bratzler shear force), water holding capacity (WHC, including cooking loss and press loss) and chroma (L*, a* and b*) of M. longissimus dorsi removed from yak within postmortem 2 h were determined during 21 d aging after vacuum packaging, and the changing patterns of quality traits of yak beef were analyzed through standardization of variables and kinetic model. It was found that there were significant variations during 21 d aging for quality traits (P<0.05) except b* value. The main variation of pH values during postmortem aging occurred in 3 d of early postmortem, while yak beef aging for 21 d did not have a lower pH value than those aging for 7 d. The aging of 7 d decreased the WBSF by 43 N, but the average WBSF of yak beef aging for 7 d still higher than 50 N. The WBSF of yak beef aging for 21 d was lower to 30 N, which was 47% significantly lower than that aging for 7 d (P<0.05). The cooking loss of yak beef significantly increased from 40.4% to 51.5% when aging time was prolonged from 7 d to 21 d (P<0.05), while the prolonged aging of 21 d did not have a significant effect on press loss of yak beef (P>0.05). The L* value significantly decreased with aging time during early aging of 7 d (P<0.05), but there was no significant difference on L* value between yak beef aging for 21 d and 7 d (P>0.05). However, a* value (redness) of yak beef significantly increased with prolonged aging time, and a* value of yak beef aging for 21 d was 32% higher compared to that aging for 7 d (P<0.05). Meanwhile, the changing pattern of quality traits of yak beef with significant change during aging was also investigated through standardization of variables and kinetic model analysis. It was found that the quadratic function had best predicting performance on quality traits of yak meat with coefficients of determination R2 ranged from 0.90 to 0.98, through comparing fitting degree of linear, exponential and quadratic functions. The changes of pH value, shearing force, press loss and L* were characterized by the convex function with negative second derivative, while cooking loss and a* value were characterized by the concave function with positive second derivative. It shows that the prolonged aging can be beneficial to improvement of tenderness, but has adverse effects on water holding capacity and meat color. Conclusively, different quality traits have different changing patterns and vary on different phases of aging.