Acidity change and storage characteristic of yougurt mixed with whey protein hydrolysate(WPH) of different hydrolytic degree (DH, 6.63 %,8.21% and 9.21%) by Papain were studied. The results showed that, WPH accelerated the aeidificadition of yogurt during fermentation, and within certain range of DH, the higher the DH, the shorter the fermentation time. Compared to the control, WPH increased the apparent viscidity of yogurt. Mo-reover, within certain range of DH, the higher the DH, the higher the apparent viscidity of yogurt. WPH with DH of 9.21% inhibited the post-acidification of yogurt, while WPH with DH of 6.63 % and 8.21% accelerated the post-acidification of yogurt.